1Heat oven to 350°F (325°F for dark or nonstick pans). Place Reynolds Baking Cups in each of 24 regular-size muffin cups. Make and cool cupcakes as directed on box for 24 cupcakes--except add vanilla to batter.
2Meanwhile, in 2-quart saucepan, melt butter over medium heat. Stir in brown sugar. Heat to boiling, stirring constantly; reduce heat to low. Boil and stir 2 minutes. Stir in milk. Heat to boiling; remove from heat. Pour mixture into medium bowl; refrigerate 1 hour or until lukewarm, about 90°F.
3Beat powdered sugar into cooled brown sugar mixture with electric mixer on low speed until smooth. If filling becomes too stiff, stir in additional milk, 1 teaspoon at a time. If filling is too soft, cover; return to refrigerator 15 minutes to firm up.
4Remove Reynolds Baking Cups from cupcakes. Using serrated knife, cut each cupcake in half horizontally, being careful not to break either half. Place heaping 1 tablespoon filling on each cupcake base. Replace rounded cupcake tops. Pipe or spoon rounded 1 tablespoon frosting onto cupcake tops. Garnish with grated chocolate. Store in airtight container at room temperature.