Preheat and Prepare: Preheat your oven to 350°F (175°C). Grease an 8-inch square baking dish with melted butter and line the bottom with parchment paper, leaving a 2-inch overhang on two sides.
Make the Crust: In a bowl, whisk together the almond flour (or ground nuts), graham cracker crumbs, sugar, and lime zest. Pour in the melted butter and mix until combined. Press the mixture into the bottom of the prepared pan, going up the sides about 1 inch. Bake the crust for 8 to 12 minutes or until lightly browned. Allow cooling for about 30 minutes.
Prepare the Filling: In a large bowl, whisk together the egg yolks and sweetened condensed milk. Add the fresh lime juice and lemon juice, whisking until smooth. Pour the mixture into the cooled crust, spreading it carefully.
Bake: Place back into the oven and bake for about 15 minutes or until the filling is set. Once baked, allow the bars to cool in the pan on a wire rack.
Chill and Cut: After cooling, refrigerate for at least 1 hour before serving. When ready, use the parchment paper overhang to lift the squares out and cut them into 16 pieces.