Preheat the oven to 325°F (165°C).
In a bowl, combine the melted butter, graham cracker crumbs, and sugar until the mixture is evenly moistened.
Press the crumb mixture firmly into the bottoms of the cavities of a lift-and-serve tart squares pan; press any remaining crumbs into two 4-inch tart pans if using.
Chill the crusts in the refrigerator for about 10 minutes, then bake at 325°F for 10–15 minutes, until just golden. Remove and let cool.
Prepare the Key lime filling (use your preferred recipe) and pour it into the cooled tart crusts.
Raise the oven temperature to 350°F (175°C). Bake the tartlets for about 12–15 minutes, until just barely set.
Cool the tartlets to room temperature, then chill before serving.
Serve with sweetened whipped cream and garnish with Key lime slices if desired.