Preheat the oven to 350 degrees F and line one 12-cup standard muffin tin with paper cupcake liners.
Cream the butter in a large bowl with an electric mixer until smooth. Gradually add the granulated sugar and beat until light and fluffy, about 3 minutes. Add the eggs, one at a time, scraping down the sides of the bowl between additions. Beat in the vanilla.
Combine the flour, baking powder, lime zest and salt in a medium bowl. Add one-third of the flour mixture to the butter mixture; beat in half the milk. Repeat additions, ending with the flour. Beat until combined, taking care not to overmix.
Divide the batter between the prepared muffin tins, filling the cups two-thirds full. Bake until a toothpick inserted into the center of the cupcakes comes out clean and the tops spring back when pressed gently, 18 to 22 minutes. Cool the cupcakes in the tins on a rack for 10 minutes, and then remove from the tins and cool completely.
To assemble: Pour the Key Lime Curd into a squeeze bottle with 1/4-inch opening. Fill the cupcakes with curd by inserting the tip into the top of each cupcake and squeezing a couple tablespoons of filling into each cupcake; the cupcake will feel heavier and curd may be visible as it comes up to the top of the cupcake.
Stir the Key Lime Frosting and dip the top of each cupcake into the frosting, allowing any excess to drip off.
Roll the edge of each cupcake in the graham cracker crumbs and place on a serving platter. Garnish with a slice of Key lime if desired.
Key Lime Curd:
Whisk together the yolks, granulated sugar, lime juice and salt in a small heavy-bottomed saucepan. Cook over medium heat, whisking constantly, about 6 minutes, or until the mixture is thick enough to coat the back of a spoon and registers 160 degrees F on an instant-read thermometer.
Remove from the heat and strain through a fine sieve into a medium bowl. Stir in the butter, one piece at a time, stirring until combined before adding the next piece. Stir in the lime zest and cover with plastic wrap pressed directly on the surface of the curd.
Chill completely, about 1 hour.
3 ounces cream cheese, at room temperature
Key Lime Frosting:
Beat the cream cheese and butter in a bowl with electric mixer at medium speed until fluffy, about 2 minutes. Gradually beat in the sweetened condensed milk, scraping down the sides of the bowl to combine well. Beat in the lime juice and vanilla. On low speed, gradually add the confectioners' sugar and beat until well combined. Chill 30 minutes.