Step 1

Arrange a rack in the middle of the oven and heat the oven to 350ºF. Lightly coat an 8x8-inch metal baking pan with cooking spray. Line the bottom and two sides with parchment paper, leaving an overhang of about 2 inches on the sides to form a sling.

Step 2

Make the crust. Place 6 tablespoons unsalted butter in a small microwave-safe bowl. Microwave in 20-second bursts until mostly melted, 40 to 50 seconds total (it will melt completely as it sits). (Alternatively, melt the butter in a small saucepan over medium heat.) Set aside to cool slightly.

Step 3

Break 12 full-size graham crackers into rough pieces and place in a food processor fitted with a blade attachment. Add 2 tablespoons granulated sugar and 1/4 teaspoon kosher salt. Process until the crackers are the texture of sand, about 15 seconds. Add the melted butter and pulse to combine, 10 to 12 pulses. (If using graham cracker crumbs, just stir the crumbs and butter together in a bowl.)

Step 4

Transfer the mixture to the prepared pan. Using your hands or the bottom of a measuring cup, press into the bottom of the baking pan in an even layer.

Step 5

Bake until fragrant and starting to darken around the edges, 12 to 15 minutes. Meanwhile, prepare the filling.

Step 6

Make the filling. Finely grate the zest of 1 to 2 medium limes until you have 3 teaspoons. Save the zested limes for another use. If needed, juice 25 to 30 key limes until you have 2/3 cup juice.

Step 7

Place 2 teaspoons of the lime zest, lime juice, 1 (14-ounce) can sweetened condensed milk, 5 large egg yolks, and 1/4 teaspoon kosher salt in a large bowl. Whisk until well-combined, smooth, and pale yellow. Pour over the crust and spread in an even layer (it’s okay if the crust is still warm).

Step 8

Bake until the filling is just set and still a little jiggly in the center, about 15 minutes. Let cool completely on a wire rack. Cover and refrigerate for at least 4 hours or up to overnight.

Step 9

Make the topping. Place 1 cup cold heavy cream in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Beat with the whisk attachment on medium speed until barely thickened, 1 to 3 minutes. Add 1 tablespoon powdered sugar and continue beating until stiff peaks form, 1 to 2 minutes more.

Step 10

Run a knife around the edges of the pan. Grasping the parchment sling, lift the key lime bar slab out of the dish and onto a cutting board.

Step 11

Spread the whipped cream over the bars. Garnish with the remaining 1 teaspoon lime zest. Cut into 16 squares and serve chilled.