eat 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.
Lower oven to 325 degrees. Zest your limes (this tool is really helpful). Then cut them in a half and juice them. In a medium bowl, gently whisk together condensed milk, egg yolks, Key lime juice, and zest. Pour into the prepared, cooled crust.
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.
Shortly before serving, combine cream and remaining 1-2 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.
You can place the pie in the freezer for up to 10 minutes before serving to help make it slice neatly.
Source: Martha Stewart