For the Cupcakes:

1. Preheat oven to 350°F (175°C). Line a 12-muffin tin with cupcake liners.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3. In a large bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy.
4. Mix in the lime zest and key lime extract.
5. Add the egg and egg yolk, beating well after each addition.
6. Reduce the mixer speed to low and alternate adding the flour mixture and sour cream, starting and ending with the flour.
7. Fold in the fresh key lime juice. The batter may look slightly curdled but will bake perfectly.
8. Divide the batter evenly among the cupcake liners, filling them about ¾ full.
9. Bake for 16-18 minutes or until a toothpick inserted in the center comes out clean.
10. Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

For the Frosting:

1. In a medium bowl, beat the butter and cream cheese until smooth and creamy.
2. Gradually add the confectioners’ sugar, mixing until light and fluffy.
3. Mix in the salt, key lime juice, key lime extract, and zest until smooth.
4. If the frosting is too thick, add a teaspoon of milk or heavy cream for a softer consistency.

To Decorate:

1. Pipe or spread the frosting onto completely cooled cupcakes.
2. Sprinkle freshly grated key lime zest over the frosting.
3. Serve immediately or store in the refrigerator for up to 4 days.