Make the lime curd: Combine the granulated sugar, egg and egg yolk, lime juice and cornstarch in a small saucepan. Cook over medium-low heat, whisking, until thick enough to coat a spoon, 8 to 10 minutes. Remove from the heat and whisk in the butter, a little at a time, until smooth. Strain through a fine-mesh sieve into a bowl, then stir in the lime zest and salt. Set in a larger bowl of ice and stir until the curd is completely cool. Press plastic wrap directly onto the surface; refrigerate until ready to use.
Make the cupcakes: Preheat the oven to 350 degrees F and line a muffin pan with 12 cupcake liners. Pulse the wafer cookies in a food processor until finely ground. Melt 2 tablespoons butter; add to the processor and pulse until the mixture comes together. Press about 1 tablespoon crumbs into each muffin cup.
Sift the flour, baking powder, baking soda and salt into a medium bowl; set aside. Combine the remaining 10 tablespoons butter and the granulated sugar in a large bowl and beat with a mixer on medium-high speed until fluffy, 5 minutes. Reduce the speed to low; beat in the eggs, one at a time. Add the vanilla, lime zest and juice and beat until well combined. Add the flour mixture in 3 additions, alternating with the buttermilk, beginning and ending with the flour mixture. Increase the speed to medium high and mix until combined.
Divide the batter among the prepared muffin cups, filling each about three-quarters of the way. Bake until a toothpick inserted into the centers comes out clean, 20 to 22 minutes. Remove to a rack; let cool completely.
Make the frosting: Beat the butter in a large bowl with a mixer on medium-high speed until fluffy, 5 minutes. Reduce the speed to low and beat in the confectioners' sugar. Increase the speed to medium high; add the sour cream, lime juice and salt and beat until fluffy, 2 more minutes.
Place the lime curd in a pastry bag fitted with a 1/4-inch tip. Insert the tip into the center of each cupcake and squeeze to fill. Frost the cupcakes, then pipe more lime curd on top. Top with lime slices and zest.
Cook's Note: Key limes are smaller and more tart than regular limes. If you can't find them, look for bottled Key lime juice and use regular lime zest.
Recipe courtesy of Food Network Magazine