Preheat the oven to 350 degrees F.  Line a cupcake pan with 12 cupcake liners.

In a large bowl, cream together the butter and sugar until light and fluffy, about 3-4 minutes.

Add sour cream, vanilla extract and and key lime zest and mix until combined.

Mix in egg whites two at a time, mixing well after each addition and scraping down the sides of the bowl as needed. 

Whisk together the flour, baking powder and salt in a separate bowl, then add alternately with the milk and key lime juice, mixing between each addition until combined and scraping the sides of the bowl as needed.  

Fill the cupcake liners about three quarters full.  Bake for 15-17 minutes, or until the top springs back with a touch or a toothpick inserted in the center of a cupcake comes out clean with just a few crumbs.  Cool completely before frosting.

Beat the butter using a hand mixer or stand mixer until smooth.

Add the powdered sugar in two batches, mixing on low speed until combined. 

Add the key lime zest and 3 tablespoons of the key lime juice, mixing until combined.  Add more key lime juice as needed to reach desired consistency.

Transfer the frosting to a piping bag fitted with a decorative tip.  Pipe the frosting onto the cupcakes, then press graham cracker crumbs around each cupcake and sprinkle with additional key lime zest, if desired, and stick a thin slice of key lime on top for decoration.