Preheat oven to 350°F (175°C) and line a 12-cup muffin pan with colorful paper liners.

Whisk together 1.5 cups of all-purpose flour, 2 teaspoons of baking powder, and 0.5 teaspoons of salt in a medium bowl.

Cream together 0.5 cups of softened unsalted butter and 1 cup of granulated sugar in a large bowl for 2-3 minutes until light and fluffy.

Incorporate 2 large eggs, mixing well after each addition, then add 1 teaspoon of pure vanilla extract, 0.5 cups of key lime juice, and the zest of 1 key lime.

Gradually mix in half of the dry ingredients followed by 0.5 cups of buttermilk and then the remaining dry ingredients until no dry flour remains.

Scoop the batter into the prepared cupcake liners, filling each about ¾ full, and bake for 18-20 minutes.

Allow the cupcakes to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.

For the frosting, beat 0.5 cups of softened unsalted butter until creamy. Gradually add 2 cups of sifted icing sugar, mix on low.

Incorporate 2 tablespoons of key lime juice and the zest of 1 lime until fluffy, adjusting consistency with a splash of cream or milk if needed.

Frost each cooled cupcake with the creamy buttercream and garnish with extra lime zest or a slice of candied lime.