Method
Prepare the oven and pan
Heat the oven to 175°C (350°F). Line a 20 x 20 cm (8 x 8 in) square baking pan (for 8 servings) with parchment, leaving enough overhang to lift out the baked slab. Lightly grease any exposed sides. For a scaled batch, choose a pan that keeps the crust and filling at a similar depth rather than scaling the dimensions directly.
Mix and bake the crust
Mix 150 g of graham cracker crumbs, 35 g of granulated sugar for crust, and 1.5 g of fine sea salt for crust in a medium bowl. Melt 85 g of unsalted butter, pour it over the crumbs, and stir until the mixture resembles evenly damp sand with no dry pockets. Press it firmly into a level layer in the prepared pan, paying particular attention to the corners. Bake for 9 to 11 minutes, starting the first check at 9 minutes, until the crust smells toasted and the edges are one shade darker. Leave the oven on and set the pan on a heatproof surface while mixing the filling.
09:00Whisk the Key lime filling
Whisk 4 large egg yolks with 6 g of Key lime zest in a large bowl until the yolks look slightly lighter and the zest is evenly dispersed. Add 397 g of sweetened condensed milk and whisk until completely smooth. Whisk in 120 ml of fresh Key lime juice and 0.75 g of fine sea salt for filling just until the mixture thickens slightly and no streaks remain. Reduce the oven temperature to 165°C (325°F).
Bake the filling until safely set
Pour the filling over the warm crust and gently tilt the pan to level it without splashing the sides. Bake for 18 to 22 minutes, beginning the first check at 18 minutes. The edges should be set, the center should move as one soft wobble rather than ripple like liquid, and a food thermometer inserted into the center should register at least 71°C/160°F. Avoid browning the top. Set the pan on a wire rack.
18:00Cool, cut, and serve
Let the bars cool in the pan on the rack for 60 minutes, until the pan is no longer warm and the filling has settled enough to lift. Use the parchment to lift out the slab. Cut it into bars according to the selected serving count with a sharp knife, wiping the blade clean between cuts. Expect softly creamy edges rather than rigid bakery-firm cuts. Serve with Key lime wedges (optional).
60:00
Smart Substitutions
- No Key limes? Use fresh Persian lime juice and zest in the same quantities. The flavor will be slightly less floral but still sharply citrusy.
- No graham crackers? Use finely crushed digestive biscuits, choosing a plain variety with a similar dry, crisp texture.
- Only salted butter? Use it in place of unsalted butter and omit the fine sea salt for crust.
- Want a milder lime edge? Use half the Key lime zest while keeping the full amount of lime juice, so the filling stays bright but less aromatic and sets as intended.
Tips and Variations
- Zest the Key limes before squeezing them; the intact fruit is much easier to hold against a fine grater.
- Crush the graham crackers to fine, even crumbs so the base cuts cleanly instead of breaking into pebbly chunks.
- Press the crust firmly enough to bind, but stop before it becomes a dense hard layer.
What Went Wrong?
- Filling cracked on top The bars baked too long or too hot. Remove them as soon as the center reaches 71°C/160°F and moves as one soft wobble.
- Crust crumbles when cut Use finer crumbs, mix until every crumb is moistened, and press the corners as firmly as the center.
- Bars are too soft to slice Check that the filling reached the safe target temperature, then allow the full 60-minute cooling period before cutting.
- Lime flavor tastes bitter Grate only the colored outer zest and leave the white pith behind; pith adds bitterness without useful citrus aroma.
Nutrition Estimate
- Fiber: 1 g per serving
- Sodium: 260 mg per serving
Nutrition disclaimer:This recipe is provided for general cooking information only. Nutrition values are estimates and may vary by ingredients, brands, preparation methods, and serving sizes. Check ingredient labels and follow appropriate food-safety practices. This content is not medical, dietary, allergy, or professional food-safety advice.
