Preheat your oven to 350°F (175°C).

Crush the graham crackers in a zip-top bag or food processor, then mix with melted butter until combined. Press this mixture into a greased baking dish.

Whisk together sweetened condensed milk, key lime juice, zest, vanilla, sour cream or yogurt, and egg yolks in a mixing bowl. Mix until smooth.

Pour the filling over the prepared crust, spreading it evenly.

Bake for about 15-20 minutes, or until the filling is just set.

Whip the heavy cream and granulated sugar together until stiff peaks form.

Spread the whipped cream over the cooled bars and chill for an hour.