To make the graham cracker crust: Preheat the oven to 350°F (180°C). Line two 12-count muffin pans with 18 liners and set aside.
Add the graham cracker crumbs and sugar to a mixing bowl and mix until well combined. Stir in the melted butter until all of the crumbs are moistened.
Evenly distribute the mixture between all 18 liners, about 1 heaping tablespoon per liner. Firmly press the graham cracker crusts down into one even layer, I like to use the bottom of a ¼ cup measuring cup for this step.
Bake for 5 minutes or until lightly browned. Remove from the oven and set aside while you make the filling.
Keep the oven temperature at 350°F (180°C).
To make the key lime pie filling: Combine the key lime juice, sweetened condensed milk, and egg yolks in a large mixing bowl and whisk until fully combined.
Evenly distribute the filling between all 18 liners in the muffin pans, filling each one almost to the top (there should still be a little room).
Bake for 12 to 15 minutes or until the tops of the pies are just set.
Remove from the oven and transfer to a wire rack. Allow to cool completely in the pans, about 1 hour. Once cooled, remove the mini pies, place them in an airtight container, and refrigerate for at least 2 to 3 hours or until set.
Top with whipped cream right before serving.
Notes
Storage Instructions: Store in an airtight container in the refrigerator for up to 4 days.Freezing Instructions: These mini key lime pies will freeze well for up to 3 months. Thaw them overnight in the refrigerator and top with whipped cream right before serving.Graham Cracker Crumbs: You can either buy full graham cracker sheets and crush them yourself, or you can save time by buying pre-made graham cracker crumbs. If crushing your own, you will need about 11 to 12 full sheets of graham crackers to get 1 and 1/2 cups of crumbs. If you live outside of the US, digestive biscuits are a great alternative!