1. Preheat the oven to 350°F and line a 12-cup muffin pan with cupcake liners.

2. In a medium bowl, whisk together the flour, baking powder, and salt.

3. In a large bowl, beat the butter and sugar until light and fluffy.

4. Add the eggs one at a time, then mix in the lemon zest, lemon juice, and vanilla extract.

5. Add half of the dry ingredients, then the milk, then the remaining dry ingredients, mixing just until combined.

6. Divide the batter evenly among the cupcake liners, filling each about two-thirds full.

7. Bake for 18 to 20 minutes, or until a toothpick inserted in the center comes out clean.

8. Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack and cool completely.

9. For the buttercream, beat the butter until creamy, then add the powdered sugar gradually.

10. Mix in the lemon juice, lemon zest, vanilla extract, salt, and enough heavy cream to make the frosting smooth and pipeable.

11. Frost the cooled cupcakes and garnish with extra lemon zest if desired.