1Heat oven to 350°F. Place paper baking cup in each of 24 regular-size muffin cups.
2In large bowl, beat cupcake ingredients with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally, until well blended. Divide batter evenly among muffin cups.
3Bake about 20 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pan to cooling rack. Cool completely, about 30 minutes.
4In medium bowl, beat butter and powdered sugar with electric mixer on medium-low speed until combined. Increase speed to medium; gradually beat in rum and vanilla until fluffy. Pipe or spread frosting on cupcakes.