Preheat the oven to 350°F (175°C)
Mix 1 1/2 cups graham cracker crumbs, 1/3 cup sugar, and 6 tablespoons melted butter
Press the mixture into a 9-inch pie dish
Bake the crust for 8 to 10 minutes
Let the crust cool slightly
Whisk together 1 can sweetened condensed milk, 1/2 cup key lime juice, and 4 egg yolks
Pour the filling into the crust
Bake for 15 minutes
Remove from the oven and let cool
Refrigerate for at least 3 hours
Top with whipped cream before serving