01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Whisk together flour, baking powder, and salt in a bowl. Set aside.
03 - Beat butter and sugar 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 creamed mixture in three additions, alternating with limoncello-milk mixture, beginning and ending with flour. Mix until just combined.
07 - Divide batter evenly among cupcake liners. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
08 - Combine water and sugar in a small saucepan. 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 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 buttercream onto cooled cupcakes. Garnish with extra lemon zest if desired.