1Heat oven to 350°F (325°F for dark or nonstick pan). Place paper baking cup in each of 24 regular-size muffin cups. In large bowl, beat cupcake ingredients with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Divide batter evenly among muffin cups, filling each about two-thirds full.
2Bake 18 to 23 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pan to cooling rack.
3In large bowl, beat cream cheese and butter on medium speed until light and fluffy. On low speed, beat in powdered sugar and Limoncello until mixed; beat on medium speed until fluffy. Frost cupcakes, mounding and swirling frosting in center.
4Garnish with lemon slices if desired. Store covered in refrigerator.