Step 1

Preheat the oven to 350°F. Line an 8x8-inch metal baking pan with parchment paper leaving an overhang of about 2 inches on the sides for easy removal.

Step 2

First make the crust. Break graham crackers into rough pieces and place in a food processor fitted with a blade attachment. Pulse until it forms fine crumbs similar to bread crumbs. Add brown sugar and melted butter and pulse again until it looks like damp sand. Transfer the mixture to the prepared pan and use your hands or the bottom of a measuring cup to press it into the bottom of the baking pan in an even layer. Bake for about 15 minutes until fragrant and starting to darken around the edges.

Step 3

Meanwhile, make the filling. Combine sweetened condensed milk, whole egg, egg yolks and vanilla into a large bowl and whisk until smooth. Mix in lime juice. Add yogurt and lime zest and mix well. Finally, mix in sugar. Gently pour the mixture over the baked crust and spread in an even layer (it’s okay if the crust is still slightly warm).

Step 4

Reduce oven temperature to 325°F and bake for 15-20 minutes until the filling is just set and still a little jiggly in the center. Let cool completely on a wire rack. Cover and refrigerate for at least 4 hours or up to overnight.

Step 5

Make the topping. Place 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 slightly thickened. Add 1 tablespoon powdered sugar and continue beating until stiff peaks form, 1 to 2 minutes more. Spread the whipped cream over the bars. 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. Grate some lime zest over top just before serving and then cut into squares. Serve chilled.