Make the Key Lime Cupcakes:

1. Preheat the oven to 350°F and prepare a cupcake pan with cupcake liners.
2. In a large mixing bowl, cream the butter and sugar together until light in color and fluffy, about 3-4 minutes. Do not skimp on the amount of creaming time.
3. Add sour cream, vanilla extract and and key lime zest and mix until well combined.
4. Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. In a separate bowl, whisk together flour, baking powder and salt.
6. In a separate small measuring cup, combine the milk and key lime juice.
7. Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
8. Fill the cupcake liners about three quarters full. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
9. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.

Make the Lime Buttercream Frosting:

1. In a large mixing bowl, combine the butter and shortening with a mixer until smooth.
2. Add about half of the powdered sugar and mix until smooth and well combined.
3. Add the key lime zest and 3 tablespoons of key lime juice and mix until combined.
4. Add the remaining powdered sugar and mix until smooth. Add additional key lime juice, as needed to reach desired consistency.
5. Pipe the frosting onto the cupcakes using a large round pastry tip, like this one.
6. Press graham cracker crumbs around the bottom edge of the frosting.