Preheat oven to 350°F (175°C) and line a 12-cup muffin pan with cupcake liners.
In a large bowl, whisk together flour, baking powder, baking soda, salt, and sugar until evenly combined.
In another bowl, mix melted butter, sour cream, eggs, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing just until combined — don’t overmix.
Divide the batter evenly among cupcake liners, filling each about ⅔ full.
Bake for 18–20 minutes or until a toothpick inserted in the center comes out clean.
Cool cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely.
For the kiwi buttercream, beat butter and shortening together until creamy and fluffy.
Add pureed kiwi and mix until fully combined.
Gradually add powdered sugar, one cup at a time, until you reach your desired frosting texture — light, smooth, and spreadable.
Pipe or spread the kiwi buttercream onto cooled cupcakes.
Garnish with a small kiwi slice or fresh mint leaf, if desired.