To prepare cupcakes, place brown sugar and butter in a large bowl; beat with a mixer at medium speed 2 minutes or until well blended. Add eggs, 1 at a time, beating well after each addition. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, cocoa, baking powder, baking soda, and salt in a bowl, stirring well with a whisk. Add flour mixture to sugar mixture alternately with buttermilk, beginning and ending with the flour mixture. Stir in vanilla extract.
Spoon batter into 18 muffin cups lined with paper liners. Bake at 350° for 12 minutes or until cupcakes spring back when touched lightly in the center. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.
To prepare frosting, combine powdered sugar, cream cheese, and peppermint extract in a bowl, stirring until smooth. Spread about 4 teaspoons frosting on each cupcake; sprinkle evenly with candies.