For the crust: Preheat oven to 375°. Butter a 9" glass pie plate with some of the melted butter, and set aside. Combine graham cracker crumbs, sugar, and remaining melted butter in a medium mixing bowl; then transfer to prepared pie plate. Spread crumbs evenly on bottom and up the side of the pie plate; then, using your fingertips, firmly press down on crumbs to form a crust. Bake crust until lightly browned, about 8 minutes. Remove crust from oven, and set on a wire rack to let cool to room temperature.
For the filling: Briefly whisk egg yolks in a large mixing bowl; then gradually add condensed milk, whisking until smooth. Add lime juice, and mix until just combined.
Pour filling into prepared crust (filling will thicken as it sits). Cover pie with plastic wrap, being careful not to let it touch the surface of the filling, and refrigerate until well chilled, at least 6 hours or overnight.
Just before serving, put heavy cream and sugar into a large, well-chilled mixing bowl. Beat cream with a whisk or an electric beater on high speed until cream just forms soft peaks (do not overbeat). Decorate pie with large dollops of whipped cream.