Tangy Lemon Slice Recipe: Sweetened Condensed Milk Traybake

This lemon slice recipe gives you a buttery shortbread base under a glossy, tangy topping that smells like sunshine. About an hour from start to oven-out, plus a chill, gets you 16 velvety squares with no mixer needed. It's my go-to for potlucks and weeknight dessert cravings, since it's made in one tin and keeps beautifully for days.

Why This Recipe Works

• Rub cold butter into flour: Cold cubes stay solid in the flour, creating tiny pockets that melt in the oven for a short, tender crumb.
• Press the base firmly before baking: Packing the crumbs tight keeps the shortbread dense and crisp, so it holds its shape under the topping.
• Acid sets the condensed milk custard: Lemon juice thickens the sweetened condensed milk while the eggs coagulate, giving a sliceable custard rather than a runny pour.
• Pull it out while still wobbly: Carryover heat finishes setting the centre after it leaves the oven, so overbaking doesn't turn the topping grainy.

Ingredients You'll Need

Plain flour: This is the structure of the shortbread base, and it gives the crumb its tender bite. Weigh it at 185 g rather than scooping by the cup, since packed flour throws the ratio off. Substitute: a 1:1 gluten-free plain flour blend, though the base may crumble a little more.

Icing sugar: Icing sugar is powdered sugar with a little starch, so it blends into the flour without grit. Sift it so no lumps end up trapped in the crumbs. Substitute: caster sugar blitzed in a food processor for about 30 seconds, which is close enough.

Cold unsalted butter: Cold cubes are what turn the flour into sandy crumbs, leaving little pockets that give the base its short snap. Keep the butter straight from the fridge and cut it into cubes before you start. Substitute: cold salted butter, with the fine salt reduced slightly since the butter already brings some.

Fine salt: A small pinch sharpens the buttery base and keeps the sweetness from tasting flat. Fine salt spreads evenly through the crumbs. Substitute: table salt in the same amount, which works just as well.

Sweetened condensed milk: This thick, sweet base gives the topping its velvety, custardy body. It's the reason this slice sets smoothly without cream or cornflour. Substitute: coconut-based sweetened condensed milk, which keeps the same thick pour, though the flavour shifts a little.

Large eggs: Eggs bind the topping and set it into a sliceable custard once baked. Beat them lightly so they blend smoothly with the milk and juice. Substitute: a commercial egg replacer made for baking, since flax eggs won't set the custard properly.

Lemon zest: Zest holds the fragrant lemon oils that give the topping its bright, sunny smell. Grate only the yellow outer layer, because the white pith turns bitter. Substitute: a little lime zest for a sharper citrus note, or a few drops of lemon extract, though you'll lose the fresh oils.

Fresh lemon juice: Juice provides the tang and helps the condensed milk thicken as the eggs set. Squeeze it fresh from about 2 lemons for the brightest flavour. Substitute: bottled lemon juice in the same 80 ml measure, which works but tastes a little flatter.

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How to Make It (Step by Step)

It all happens in one lined tin: a crumbly shortbread base, a zesty condensed milk topping, then a chill. Even if you've never baked a traybake before, you'll get golden edges and a wobbly centre without fuss.

1. Heat the oven to 180°C / 350°F and line a 20 cm square tin with baking paper, leaving overhang for lifting, until the oven is fully hot, 10-15 minutes at 180°C / 350°F.

2. Rub the cold cubed butter into the flour, icing sugar and salt until sandy crumbs form with no large lumps, 3-5 minutes at 18-20°C / 65-68°F.

3. Press the crumbs firmly into the lined tin and bake until the edges turn light golden-brown and the surface looks dry, 15-18 minutes at 180°C / 350°F.

4. Whisk the condensed milk, lightly beaten eggs, lemon zest and lemon juice until smooth, glossy and pale yellow, 1-2 minutes at 20°C / 68°F.

5. Pour the topping over the warm base and bake until just set with a slight wobble in the centre, 20-25 minutes at 180°C / 350°F.

6. Cool the slice in its tin on a wire rack until it stops jiggling and feels firm to a gentle press, 45-60 minutes at 20°C / 68°F.

7. Cover the tin and chill until the slice is firm and cold to the touch, 2-3 hours at 4°C / 39°F.

8. Lift the slice out with the paper overhang and cut into 16 squares with a sharp knife, wiping the blade between cuts, until edges are clean and firm, 2-3 minutes at 4°C / 39°F.

Recipe Notes

• Storage: Keep the slice covered in the fridge for up to 5 days. Freeze cut squares wrapped individually for up to 3 months. Thaw overnight in the fridge and eat them chilled.
• Key substitution: No icing sugar? Blitz caster sugar in a food processor for 30 seconds, then sift it. Use the same 60 g. The base will be slightly grittier but just as tender.
• Make-ahead tip: The whole slice can be made up to 2 days ahead. Keep it chilled and covered, and cut it just before serving so the edges stay neat.
• Common mistake: Reaching for evaporated milk instead of sweetened condensed milk. Evaporated milk is unsweetened and won't set into a sweet, tangy custard, so check the label first.

Serving size: 1 portion · Calories: 215 · Total Fat: 11g · Saturated Fat: 7g · Carbs: 26g · Fiber: 0g · Sugar: 19g · Protein: 4g · Sodium: 95mg

Tips and Tricks

• Equipment: Use a 20 cm square tin: Line it with baking paper, leaving overhang on two sides. You'll lift the whole slice out and cut it cleanly.

• Temperature precision: Bake the topping at 180°C / 350°F: Start checking at 20 minutes. The centre should wobble gently, not slosh, before you pull it out.

• Common mistake: Whisking the eggs too hard: Whisk until just smooth. Vigorous whisking traps air, leaving small pockmarks across the glossy top.

• Flavour upgrade: Add a pinch of extra zest to the base: Rub a little lemon zest into the flour and icing sugar. The base smells brighter and tastes fresher.

• Texture tip: Use a hot, dry knife for clean edges: Dip the blade in hot water, dry it, then cut. Each square stays velvety, not smeared.

Frequently Asked Questions

Can I make this lemon slice gluten-free or dairy-free without changing the texture?

Swap the 185 g plain flour for an equal weight of a 1:1 gluten-free plain flour blend that contains xanthan gum. For dairy-free, use 125 g firm plant-based butter block and a 395 g can of coconut sweetened condensed milk, though the topping may set slightly softer.

Can I freeze the shortbread base before adding the topping?

Yes, press the cold base into the lined tin, wrap it well and freeze for up to 1 month. Bake it straight from frozen at 180°C / 350°F for 20-25 minutes until golden, then add the topping as usual.

Why does my lemon slice topping crack or develop bubbles on top?

Over-whisking traps air in the condensed milk mixture, and an oven running hot makes the surface crack. Whisk only until smooth, tap the filled tin on the counter to release bubbles, and check oven temperature with a thermometer. Look for an even, glossy top.

What to Serve With This

• Fresh Raspberries: Their sharp tartness cuts straight through the sweet, velvety condensed milk topping.
• Plain Greek Yoghurt: Cool, unsweetened tang gives a creamy contrast to the rich, buttery shortbread.
• Passionfruit Pulp: Tropical sourness adds bright, seedy bursts that pick up the zing already in the slice.
• Strong Black Coffee: Its bitter edge balances the sugary sweetness, so every bite feels lighter.