Stir together graham cracker crumbs, sugar, and butter in a small bowl.
Press evenly onto bottom of an 8-inch square glass baking dish.
Bake until dry and golden brown, about 10 minutes. Let cool completely on a wire rack.
Put egg yolks and lime zest in the bowl of an electric mixer fitted with the whisk attachment. Mix on high for 3 minutes to thicken. Reduce speed to medium and add condensed milk in a slow stream, mixing constantly. Raise speed to high; mix until thick, about 3 minutes. Reduce speed to low and add lime juice; mix until just combined.
Spread filling over baked crust using a spatula. Bake, rotating dish halfway through, until filling is just set, about 10 minutes. Let cool completely on a wire rack before refrigerating for at least 4 hours.
After the tray is chilled, cut into bars. Put cream in the bowl of an electric mixer fitted with the whisk attachment. Mix on medium-high speed until stiff peaks form. Garnish bars with whipped cream and a little bit of extra lime zest
Recipe slightly adapted from Martha Stewart Holiday Cookies, 2005