To make the cupcakes:

Cup of Yum
1. Preheat the oven to 350ºF. Line 36 muffin tins with cupcake liners.
2. In a bowl, combine the flour, baking powder and salt. Set aside.
3. In a large bowl, or the bowl of a stand mixer, beat the sugar and the butter together on medium speed until combine.  Add in the eggs, one at a time, scraping down the sides of the bowl as needed. Add the the zest and the vanilla and beat to combine.
4. With the mixer on low speed, add 1/3 of the flour mixture. When it’s almost mixed in, add half of the buttermilk, followed by an additional 1/3 of the flour, and then the final buttermilk. Then mix in the remaining flour. Continue to beat just until it is blended.
5. Divide the batter evenly between the prepared muffin tins, filling each about 2/3 full.
6. Bake until a tester inserted in the middle of the cupcakes comes back with just a few crumbs, about 15 to 18 minutes. Cool in the pans for 5 minutes before removing to a cooling rack to cool completely.

To make the filling:

1. In a bowl, beat the cream cheese until smooth.
2. Add the sweetened condensed milk and lime juice and beat to combine.
3. Reserve 1/2 cup of the filling for the buttercream, and refrigerate the remainder until ready to use.

To make the buttercream:

1. Beat the butter in a large bowl until light.
2. Add in 3 cups of the powdered sugar, the reserved key lime filling, the lime juice, vanilla and salt and continue to beat until combined.
3. Gradually add more powdered sugar, 1/2 cup at a time, until the buttercream is smooth and thick.

To make the graham cracker crumble:

1. Preheat the oven to 350ºF.
2. Line a baking sheet with foil.
3. In a small bowl, combine the graham cracker crumbles, the melted butter and the brown sugar.
4. Spread out on the baking sheet into a thin layer.
5. Bake for 4 to 6 minutes, just until golden, then remove from the oven.
6. Allow it to cool completely, then crumble into small pieces.

To assemble the cupcakes:

1. When the cupcakes are cool, use a sharp knife to cut a cone out of the center of each cupcake, reserving the cut piece. (I will usually cut off the cone end and discard that, leaving just the circle to cover the filling once filled.) Fill the cupcake with some of the key lime filling, then replace the cut out piece back onto the cupcake.
2. Pipe the buttercream onto the cupcakes, then garnish with the graham cracker crumbles.