These bars have two layers: a pressed graham cracker crust and a key lime custard made from egg yolks, sweetened condensed milk and lime juice. They bake in an 8 by 8 inch pan for 25 minutes in total and then need at least 2 hours in the fridge to set.
The custard sets from the lime juice and egg yolks together. That is why the center should still wobble when it comes out of the oven; it firms up as it chills.
Ingredients (one 8 by 8 inch pan)
Crust
- 1 1/2 cups graham cracker crumbs (about 10 full sheets)
- 1/3 cup granulated sugar
- 1/2 cup unsalted butter, melted
Filling
- 3 large egg yolks
- 1 can (14 ounces) sweetened condensed milk
- 1/2 cup key lime juice, fresh (15 to 20 key limes) or bottled key lime juice
- 1 tablespoon lime zest
- A pinch of salt
Crust, filling, chill
Crust, 10 minutes in the oven. Heat the oven to 350°F and line the pan with parchment, leaving an overhang on two sides for lifting. Stir the crumbs, sugar and butter until the mix looks like damp sand and holds together when pinched. Press it firmly and evenly into the pan with the bottom of a measuring cup and bake 10 minutes.
Filling, 15 minutes in the oven. Whisk the yolks for about a minute until lighter in color. Whisk in the condensed milk, lime juice, zest and salt until smooth. Pour it over the warm crust, tilt the pan to level it, and bake 15 minutes. The edges should look set and the center should still jiggle.
Chill, at least 2 hours. Cool the pan on the counter for 30 minutes, then refrigerate for at least 2 hours, or overnight. Lift the slab out by the parchment and cut with a sharp knife, wiping it clean between cuts.
After chilling, the bars should spend no more than 2 hours out of the fridge, as USDA FSIS advises.
If something looks wrong
| What you see | Why | Next time |
|---|---|---|
| Crust crumbles when cut | Not packed firmly, or not chilled long enough | Press harder; freeze the slab 30 minutes before cutting |
| Filling is rubbery | Baked until the center was firm | Check at 14 minutes and pull it while it wobbles |
| Filling tastes sharp or flat | Regular limes taste sharper and less floral; old bottled juice loses flavor | Use key lime juice and add another teaspoon of zest |
| Crust tastes too sweet | The filling is already sweet | Cut the crust sugar to 1/4 cup |
How you cut them changes the numbers
A little more than half the calories come from the crust: about 134 per bar when the pan is cut into 12, against about 122 from the filling. Using USDA values for the whole pan:
| Cut the pan into | Calories per bar | Protein per bar |
|---|---|---|
| 9 bars | about 342 | 5.5 g |
| 12 bars | about 257 | 4.1 g |
| 16 bars | about 192 | 3.1 g |
Cutting the butter in the crust to 6 tablespoons takes off about 17 calories per bar in a 12 cut pan, and the crust still holds together if you press it firmly.
At 12 bars per pan, each bar has about 257 calories, 4 grams of protein, 13 g fat and 33 g carbohydrate.
In the fridge and freezer
Keep the bars covered in the fridge for up to 3 to 4 days. They cut more cleanly on the second day. To freeze, set them on a parchment-lined sheet until solid, then stack them in a zip-top bag with parchment between layers for up to 2 months. Thaw in the fridge overnight.
Ingredient protein figures are based on USDA FoodData Central. How we calculate nutrition.
