Using a fork, stir together the crust ingredients in a medium bowl until fully combined and the graham cracker crumbs are moist.
Press the mixture into a 9-inch pie dish. Using your hands or a measuring cup, press and pat down the bottom and sides.
Chill for 20-30 minutes. I prefer to put it in the freezer for 20 minutes.
Preheat the oven to 350 degrees F.
Combine condensed milk, sour cream, and egg yolks in a medium bowl. Whisk together on medium speed until thoroughly combined.
Add (key) lime juice and (key) lime zest. Whisk until thoroughly combined.
Pour the mixture into the prepared crust.
Bake for 15-17 minutes in the preheated oven. Do NOT brown the pie; only the crust should start turning a nice golden-brown color.
Remove from the oven and cool on a cooling rack. When cool, transfer it to the fridge. Chill the pie thoroughly in the refrigerator for 3-4 hours before serving.
Serve cold, and do not keep at room temperature for more than one hour.
In a large bowl, beat heavy cream, sugar, and vanilla extract until stiff peaks form.
Add the whipped cream to a piping bag fitted with a star tip and pipe it on top of the pie, or use a spatula to spread it onto the pie.
Garnish with (key) lime slices, (key) lime zest, and whipped cream if desired.
