Step 1

Heat oven to 375 degrees. Combine graham-cracker crumbs, butter, and 3 tablespoons sugar in a medium bowl; mix well. Press into a 9-inch pie plate, and bake until lightly browned, about 12 minutes. Remove from oven, and transfer to a wire rack until completely cooled.

Step 2

Lower oven to 325 degrees. In a medium bowl, gently whisk together condensed milk, egg yolks, Key lime juice, and zest. Pour into the prepared, cooled crust.

Step 3

Return pie to oven, and bake until the center is set but still quivers when the pan is nudged, 15 to 17 minutes. Let cool completely on a wire rack.

Step 4

Shortly before serving, combine cream and remaining 2 tablespoons sugar in the bowl of an electric mixer fitted with a whisk attachment. Whisk on medium speed until soft peaks form, 2 to 3 minutes. Spoon over cooled pie; garnish with zest. Serve immediately.