No buttermilk? Use one tablespoon vinegar of lemon juice plus enough milk to make one cup.
Preheat oven to 350*. Beat together cream cheese, powdered sugar, and egg until smooth. Stir in the chopped candy bars, and set aside. In a separate bowl, mix flour, sugar, cocoa, baking soda, and salt. Add in butter milk, oil, vanilla, and mix well. Pour into cupcake pan, up to about half full. Spoon in 1 Tablespoon of the cream cheese mixture into the center of batter in each cup. Bake for 23 to 30 minutes, and cool in pans for 15 minutes. Then cool on wire racks. In a saucepan, cook brown sugar and butter until mixture begins to boil. Remove from heat and stir in milk. Let cool for 30 minutes. Then beat in the powdered sugar until it becomes spreadable (keep adding an additional Tablespoon of powdered sugar if necessary). Frost cupcakes, and sprinkle on the last candy bar.