Preheat oven to 350 degrees F. Stir graham cracker crumbs, sugar, and melted butter until the texture of wet sand. Divide among 6 lined muffin cups, pressing firmly with the bottom of a small glass or measuring cup to create an even layer. Bake until fragrant and set, 8-10 minutes. Let cool completely.

Whisk egg yolks and lime zest in a medium bowl until pale and slightly thickened, about 2 minutes. Add sweetened condensed milk and whisk until smooth, then stir in key lime juice. The mixture will thicken slightly. Strain through a fine mesh strainer to remove any zest clumps or egg bits.

Divide filling among cooled crusts, filling almost to the top. Bake until the edges are set but the centers still jiggle slightly when nudged, 10-12 minutes. Do not overbake - they will firm up as they cool. Let cool at room temperature for 30 minutes, then refrigerate at least 2 hours until fully chilled.

Beat cold heavy cream and powdered sugar to soft peaks. Dollop or pipe onto chilled pies just before serving.