This recipe is based on the famous Key-lime pie from Joe's Stone Crab restaurant in Miami Beach.
Preheat oven to 350 degrees. Stir together crumbs, sugar, and butter in a small bowl. Press evenly onto bottom of an 8-inch square glass pan. Bake until dry and golden brown, about 10 minutes. Let cool in pan on a wire rack. Leave oven on.
Place egg yolks and zest in the bowl of an electric mixer fitted with the whisk attachment; beat on high speed until mixture is very thick, about 5 minutes. On medium speed, pour in condensed milk in a steady stream, scraping down sides of bowl. Beat on high speed until thick, about 3 minutes. On low speed, add lime juice; mix until just combined.
Pour filling over crust; spread evenly. Bake until just set, about 10 minutes. Let cool completely on a wire rack. Chill at least 4 hours or overnight.
Beat cream in a medium bowl until stiff peaks just form. Cut dessert into 1 1/2-by-2-inch rectangles. Garnish each with whipped cream and a slice of Key lime.
If you can't find Key limes, use regular fresh lime juice. The bars will keep, wrapped in plastic, in the refrigerator for up to 3 days.