Molly's Amazing Chocolate Cupcakes with Peanut Butter Buttercream Frosting

CAKE:

3 oz. Unsweetened Chocolate
½ cup water
1 cup Sour Cream
1-3/4 cups cake flour
1-1/2 tsp. Baking Powder
1 tsp. baking soda
½ tsp. salt
2/3 cup butter, softened
1 cup granulated sugar
2/3 cup firmly packed brown sugar
3 eggs
2 tsp. vanilla

Preheat oven to 350°F. Grease and flour paper liners in 2 12-count muffin pans. Set aside. Microwave chocolate and water in medium microwaveable bowl on HIGH 1 to 2 min. or until chocolate is almost melted, stirring after each min. Stir until chocolate is completely melted; cool completely. Stir in sour cream.
Mix flour, baking powder, baking soda and salt; set aside. Beat butter and sugars in large bowl with electric mixer on medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add flour mixture alternately with the chocolate mixture, beating after each addition until well blended. Add vanilla; mix well. Pour evenly into prepared tins.
Bake 15 to 18 min. or until toothpick inserted in centers comes out clean. Cool cupcakes in pans 10 min.; remove from pans. Cool completely on wire racks.
FROSTING:
3 cups powdered sugar
1/3 cup butter or margarine, softened
1 1/2 tsp. vanilla
1/4 cup milk
1/4 cup peanut butter

Stir together powdered sugar and butter in medium bowl. Stir in vanilla, milk and peanut butter. Beat until smooth. Pipe onto cupcakes using pastry bag.