Preheat your oven to 350°F. In a food processor, pulse the graham cracker sheets until they turn into fine crumbs. Add the melted butter and pulse again until combined. If you don't have a food processor, place the crackers in a large ziplock bag and crush them with a rolling pin, then mix in the melted butter in a bowl.

Press the crumb mixture firmly into the bottom and up the sides of a 9-inch pie dish, using the back of a measuring cup or a small glass. Bake for 8 minutes, then set aside to cool.

In a medium bowl, whisk together the sweetened condensed milk, egg yolks, key lime juice, lime zest, and sour cream until smooth. Pour the filling into the cooled graham cracker crust and bake for 10 minutes.

Let the pie cool to room temperature, then refrigerate for at least 4 hours, or overnight, to allow it to set completely.

In a large bowl, whip the cold heavy cream with an electric mixer until soft peaks form. Gently fold in the powdered sugar. Serve each slice of pie with a dollop of whipped cream, or pipe the cream over the entire pie.