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 on a wire rack. (Leave oven on).
MAKE FILLING: Put egg yolks and lime zest in the bowl of an electric mixer fitted with the whisk attachment. Mix on high speed until very thick, about 5 minutes. Reduce speed to medium. Add condensed milk in a slow steady stream, mixing constantly. Raise speed to high; mix until thick, about 3 minutes. Reduce speed to low. Add lime juice; mix until just combined.
Spread evenly over crust using a spatula. Bake rotating dish halfway through, until filling is just set, about 10 minutes. let cool completely on a wire rack. Refrigerate at least 4 hours (or overnight),.
Cut into 2-by-2-inch bars. Put cream in the clean bowl of an electric mixer fitted with the clean whisk attachment.
Mix on medium-high speed until stiff peaks form.
Garnish bars with whipped cream and a 1/2 slice of lime.
Ungarnished, bars can be refrigerated in an airtight container up to 3 days.