Preheat oven to 350 degrees F. Lightly grease a mini tart pan or line a muffin pan with cupcake liners.

In a medium bowl, stir together the graham cracker crumbs and sugar. Pour in the melted butter and mix until the crumbs look like wet sand and hold together when pressed.

Scoop the crust mixture into the tart pan or muffin cups. Press the crumbs firmly into the bottom and slightly up the sides using the bottom of a glass or spoon.

Bake the crusts for 5 to 7 minutes until lightly golden. Remove from the oven and cool slightly.

In a large bowl, beat the softened cream cheese until smooth and creamy with no lumps remaining.

Add the sweetened condensed milk and mix until fully combined.

Pour in the key lime juice, lime zest, egg yolks, and vanilla extract. Mix until smooth and creamy.

Spoon the filling evenly into the prepared tart shells. Smooth the tops gently with a spoon.

Bake for 10 to 12 minutes until the edges are set and the centers jiggle slightly.

Remove the tartlets from the oven and cool at room temperature for 30 minutes.

Transfer the tartlets to the refrigerator and chill for at least 2 hours until fully set.

Top with whipped cream, lime zest, or lime slices before serving.