In a large bowl, beat the sugar, buttermilk, oil and vanilla until well blended. Combine the flour, cocoa, baking soda and salt; gradually beat into sugar mixture until blended.
For filling, in a small bowl, beat cream cheese and confectioners' sugar until light and fluffy. Add egg; mix well. Stir in the candy bars.
Fill paper-lined muffin cups one-third full with batter. Drop filling by tablespoonfuls into the center of each cupcake (cups will be about half full). Bake at 350° for 20-25 minutes or until a toothpick inserted in the filling comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For frosting, in a small saucepan, melt butter. Stir in brown sugar. Bring to a boil; cook for two minutes, stirring occasionally. Remove from the heat; stir in the milk, then confectioners' sugar. Cool until frosting reaches spreading consistency. Frost cupcakes.
1 cupcake equals 250 calories, 9 g fat (3 g saturated fat), 23 mg cholesterol, 248 mg sodium, 40 g carbohydrate, 1 g fibre, 4 g protein.