01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Whisk flour, baking powder, and salt together in a medium bowl. Set aside.
03 - Beat softened butter and granulated sugar in a large bowl with a mixer until light and fluffy, about 2 to 3 minutes.
04 - Beat in eggs one at a time, ensuring each is fully incorporated. Stir in lemon zest, lemon juice, and vanilla extract.
05 - Add half of the dry ingredients to the wet mixture and mix until just combined. Pour in whole milk, blend, then add the remaining dry mixture. Mix gently until smooth, avoiding over-mixing.
06 - Divide the batter evenly among the prepared liners, filling each about two-thirds full.
07 - Bake for 16 to 18 minutes until a toothpick inserted into the center comes out clean. Allow cupcakes to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
08 - Beat softened butter until smooth, then gradually add powdered sugar. Mix in lemon juice, lemon zest, and a pinch of salt. Continue beating until light and fluffy.
09 - Once cupcakes are fully cooled, decorate with lemon buttercream using a piping bag or spatula.