Preheat the oven to 350?F (175?C) and line a muffin tin with cupcake liners.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir in the buttermilk, key lime juice, and vanilla extract until combined.

In another bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Divide the batter evenly among the cupcake liners, filling each about 2/3 full.

Bake for 15-18 minutes or until a toothpick inserted into the center comes out clean.

Allow the cupcakes to cool completely on a wire rack.

In a large mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, mixing well until combined.

Add the key lime juice and vanilla extract, and beat until smooth and fluffy.

If the frosting is too thick, add a tablespoon of milk to reach the desired consistency.

Frost the cooled cupcakes with the buttercream frosting using a spatula or piping bag.

Sprinkle lime zest on top for garnish.