Got it from Betty Crocker website....
1 box Betty Crocker® SuperMoist® chocolate fudge cake mix
1 1/3 cups water
1/2 cup vegetable oil
1 teaspoon vanilla
Filling and Garnish
1 cup butter or margarine
2 cups packed brown sugar
1/2 cup milk
4 cups powdered sugar
1 oz grated semisweet baking chocolate, if desired
1. Heat oven to 350°F for shiny metal pans (or 325°F for dark or nonstick pans). Spray bottoms only of 24 regular-size muffin cups with baking spray with flour.
2. In large bowl, beat cake mix, water, oil, eggs and vanilla with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Divide batter evenly among muffin cups.
3. Bake 18 to 24 minutes or until toothpick inserted in center comes out clean. Cool 15 minutes; remove from pan to cooling rack. Cool completely, about 30 minutes.
4. Meanwhile, 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 about 30 minutes or until lukewarm.
5. Beat powdered sugar into cooled brown sugar mixture on low speed until smooth. If frosting becomes too stiff, stir in additional milk, 1 teaspoon at a time.
6. 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.
High Altitude (3500-6500 ft): Follow High Altitude directions on cake mix box for cupcakes, adding vanilla to batter.