Preheat your oven to 350°F (175°C). In a food processor, finely grind the graham cracker sheets into crumbs. Incorporate the melted butter and pulse until well combined. Alternatively, crush crackers in a sealed bag with a rolling pin and stir in melted butter.

Firmly press the mixture into a 9-inch pie pan, using the back of a measuring cup or a small glass to create an even layer. Bake the crust for 8 minutes, then set it aside to cool completely.

In a medium mixing bowl, whisk together the sweetened condensed milk, egg yolks, Key lime juice, Key lime zest, and sour cream until the mixture is smooth and fully blended. Pour this vibrant filling into your cooled graham cracker crust.

Bake the pie for 10 minutes. After baking, remove the pie from the oven and allow it to cool on a wire rack at room temperature before transferring it to the refrigerator. It is crucial to chill the pie for a minimum of 4 hours, or ideally overnight, to ensure it sets properly before slicing.

In a large, chilled bowl, whip the cold heavy cream using an electric mixer until soft peaks begin to form. Gently fold in the confectioners' sugar until just combined. Serve each slice of pie with a generous dollop of this fresh whipped cream, or pipe it decoratively over the entire pie before serving.