• Preheat oven to 350 degrees.
• In a bowl, beat the cake mix, sour cream, butter, eggs, and vanilla together on low speed with an electric mixer just until dry ingredients are moistened. Increase speed to medium, and beat 3 to 4 minutes or until smooth.
• Place paper baking cups in muffin pans, and coat with cooking spray; spoon batter evenly into baking cups, filling each two-thirds full.
• Bake at 350 degrees for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove cupcakes from pans to wire racks, and cool completely.

To prepare the frosting:

• In a bowl, beat butter and cream cheese together with a mixer until creamy. Gradually add powdered sugar, beating at low speed until blended. Increase speed to medium, and slowly add milk and vanilla, beating until smooth.
• In a microwave-safe bowl, microwave dark chocolate chips on 50% power for 1 1/2 to 2 minutes or until melted and smooth, stirring at 30-second intervals. Gradually add melted chocolate to butter mixture and beat until blended and smooth. Makes 24 cupcakes.