Peanut Butter Cupcakes with Chocolate Frosting

These are truly delectable. I made them for an outdoor party in Marin yesterday and they were gobbled up. I think I'll have to make another batch. You can see in this photo there were a few "tunnels" in the cupcakes which supposedly means I over-mixed them, but who cares?

Peanut Butter Cupcakes (from The Fannie Farmer Baking Book by Marion Cunningham)
6 Tbsp. butter
1/2 cup creamy peanut butter
1-1/4 cups brown sugar
2 eggs
1 tsp. vanilla extract
2 cups cake flour
2 tsp. baking powder
1/2 tsp. salt
1 cup milk
1 cup salted peanuts, chopped (I didn't include these because I didn't have them)

Preheat oven to 350 degrees. Line muffin tins with fluted paper baking cups. Cream the butter and peanut butter together, then add the brown sugar gradually, beating until well blended. Add the eggs and vanilla and mix well. Sift the cake flour, baking powder and salt together onto a piece of waxed paper. Add the sifted dry ingredients and the milk to the peanut butter mixture. Beat until the batter is thoroughly blended and perfectly smooth. Stir in 3/4 cup salted peanuts, if desired, reserving the rest to garnish the frosted cupcakes. Spoon into the prepared muffin tins, filling each cup about 3/4 full. Bake for about 20 minutes, until a toothpick inserted into the center of a cupcake comes out clean. Turn out onto a rack to cool. Frost with Chocolate Butter Frosting, below, and sprinkle with reserved nuts.
(makes about 17 cupcakes)

Chocolate Butter Frosting
(makes 1 cup)
1-1/2 oz. unsweetened chocolate
3 Tbsp. milk
1-1/2 cups sifted confectioners' sugar
3 Tbsp. butter, softened
dash of salt
1 tsp. vanilla

Melt the chocolate in a double boiler or in the microwave oven, then set aside. Heat the milk up in the microwave, separately, then pour it over the sugar in a mixing bowl, and beat vigorously until smooth. Add the melted chocolate and beat well. Let the mixture come to room temperature, then beat in the butter, salt and vanilla. Spread on the cupcakes.