Heat oven to 350°F. Place paper baking cups in each of 18 regular-size muffin cups. In small bowl, beat cream cheese, 2 tablespoons powdered sugar and the egg with electric mixer on medium speed until smooth. With spoon, stir in 2 chopped candy bars; set aside.
In large bowl, mix flour, granulated sugar, cocoa, baking soda and salt. Add buttermilk, oil and vanilla; beat 2 minutes with mixer on medium speed. Divide batter evenly among muffin cups, filling each half full. Spoon 1 tablespoon cream cheese mixture in center of batter in each cup.
Bake 23 to 30 minutes or until cream cheese mixture is light golden brown. Cool in pans 15 minutes. (Cupcakes will sink slightly in center.) Remove cupcakes from muffin cups. Cool completely, about 30 minutes.
Meanwhile, in 1 1/2-quart saucepan, cook brown sugar and butter over medium heat just until mixture boils, stirring frequently. Remove from heat. Stir in milk. Cool 30 minutes. With spoon, beat 1 1/2 cups powdered sugar into brown sugar mixture until spreading consistency, adding 1 tablespoon additional powdered sugar at a time if necessary.
Frost cooled cupcakes. Sprinkle with chopped candy bar.