1Heat oven to 350°F (325°F for dark or nonstick pan). Place paper baking cup in each of 12 regular-size muffin cups.
2In large bowl, beat all 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.
3Bake 18 to 23 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes; remove from pan to cooling rack. Cool completely, about 1 hour.
4Meanwhile, in 2-quart heavy saucepan, melt 1/4 cup butter over medium heat. Stir in brown sugar with whisk. Heat to boiling, stirring constantly. Stir in milk. Return to boiling. Remove from heat; cool until lukewarm, about 30 minutes. Stir in 1/2 teaspoon vanilla; gradually stir in powdered sugar until spreadable (add additional milk if frosting becomes too thick). Frost cupcakes. Sprinkle with sea salt.