01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a bowl, whisk together flour, baking powder, and salt. Set aside.
03 - In a large bowl, beat the butter and sugar together on medium speed until light and fluffy, about 2-3 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in lemon zest and vanilla extract.
05 - Combine limoncello, milk, and lemon juice in a small bowl.
06 - Add flour mixture to the creamed mixture in three additions, alternating with the limoncello-milk mixture, beginning and ending with flour. Mix until just combined.
07 - Divide the batter evenly among the cupcake liners. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
08 - In a small saucepan, combine water and sugar. Bring to a simmer, stirring until sugar dissolves. Remove from heat and stir in limoncello.
09 - Let cupcakes cool 5 minutes in the pan. Brush tops with the limoncello syrup. Transfer to a wire rack to cool completely.
10 - Beat butter until creamy. Gradually add powdered sugar, lemon zest, and salt. Beat in limoncello and lemon juice until smooth and fluffy.
11 - Pipe or spread the buttercream onto cooled cupcakes. Garnish with a little extra lemon zest, if desired.