Key Limes are smaller than their larger cousins -- the standard lime -- and also sweeter, giving these cupcakes a tangy-sweet touch. Store the cupcakes in airtight containers for up to 2 days at room temperature, or frozen up to 2 months.
Preheat oven to 350 degrees. Line two standard 12-cup muffin tins with paper liners. In a food processor, pulse together graham cracker crumbs, 5 tablespoons melted butter, 1/4 cup sugar and 1/4 teaspoon salt until mixture holds together when squeezed. Spoon 1 to 2 tablespoons crumb mixture into each liner and press firmly using the bottom of a lightly oiled shot glass or small bowl. Bake until set, 5 to 7 minutes; cool completely. Reduce oven to 325 degrees.
In a medium bowl, whisk together flour, baking powder, and remaining 1 teaspoon salt. Using an electric mixer, beat together remaining 1 cup butter and 1 cup plus 2 tablespoons sugar on medium-high speed until pale and fluffy, at least 8 minutes. Add eggs, one at a time, beating thoroughly between each addition, and scraping down sides of bowl as needed. Beat in vanilla and zest. Reduce speed to low; add flour mixture in 2 batches, alternating with sour cream and beating until just incorporated.
Divide batter among cups, filling each about three-quarters full; tap pans on countertop once to evenly distribute batter. Bake until a cake tester inserted in centers comes out clean, about 26 minutes. Transfer tins to wire racks to cool 10 minutes; turn out cupcakes onto racks and let cool completely, about 30 minutes. Frost with 2 heaping tablespoons key lime frosting; top with lime zest.