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.
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.
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 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.