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.

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 before frosting.

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

Gradually add the powdered sugar, mixing on low speed until combined.

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

If the frosting is too thick, add milk a little at a time until desired consistency is reached.

Once the cupcakes are completely cool, use a spatula or piping bag to frost each cupcake with the buttercream.

Optional: Garnish with lime zest or a slice of lime for decoration.