Crust Time: Preheat your oven to 350°F (175°C). In a medium bowl, combine the graham cracker crumbs and sugar. Pour in the melted butter and stir until well combined.
Press It In: Divide the graham cracker mixture evenly among the 12 muffin cups. Use the back of a spoon or your fingers to firmly press the crumbs into the bottom and slightly up the sides of each cup.
Pre-Bake: Bake the crusts for 8-10 minutes, until lightly golden. Remove from the oven and let them cool slightly while you make the filling.
Lime Love: In a separate bowl, whisk together the sweetened condensed milk, key lime juice, egg yolks, and key lime zest (if using) until smooth.
Fill 'Em Up: Pour the key lime filling evenly into the pre-baked crusts.
Bake Again: Return the muffin tin to the oven and bake for another 12-15 minutes, or until the filling is just set (it should jiggle slightly in the center).
Chill Out: Remove the mini pies from the oven and let them cool completely in the muffin tin. Then, transfer the tin to the refrigerator and chill for at least 2-3 hours, or preferably overnight, to allow them to fully set.
Whip It Good (Optional Topping): Just before serving, if you're feeling fancy, whip the cold heavy cream with the powdered sugar until soft peaks form.
Serve & Savor: Carefully remove the mini pies from the muffin tin. Top each one with a dollop of whipped cream and a slice of lime or a sprinkle of zest for that final sassy touch. Prepare for a burst of flavor!