Preheat oven to 350 degrees and line muffin tin with baking cups (24)
Allow butter and eggs to stand at room temperature for 30 minutes.
In a medium bowl stir together flour, baking powder, baking soda, and salt and set aside.
In a large bowl beat butter with an electric mixer on medium to high speed for 30 seconds.
Add sugar and lemon peel; beat until combined.
Add eggs, one at a time, beating well after each addition.
Alternately add flour mixture and milk mixture (including the limoncello, if using) to butter mixture, beating on low speed after each addition just until combined.
Spoon batter into prepared muffin cups, filling each about three-fourths full.
Bake about 18 minutes or until tops spring back when lightly touched. Cool cupcakes in muffin cups on wire racks for 5 minutes.
Remove cupcakes from muffin cups. Cool completely on wire racks.
Frost with Lemon Frosting recipe below.
Top with lemon slices rolled in sugar.
FROSTING
Allow butter to stand at room temperature for 30 minutes.
In a large mixing bowl beat butter and extract until smooth.
Slowly add 1 cups of the powdered sugar at a time, beating well after each addtion.
Add lemon juice and zest. Mix well.
Beat in milk, 1 teaspoon at a time, until frosting reaches spreading consistency.