Preheat oven to 350ºF. Place baking cups in muffin pan.
In medium bowl, combine flour, baking soda and baking powder.
In large bowl, beat 1/4 cup butter, 1/2 cup peanut butter and sugar with an electric mixer until light and fluffy. Add eggs, sour cream and 1 teaspoon vanilla; beat until well combined. Alternately add flour mixture and 2/3 cup plus 1 tablespoon milk to bowl, beating until just combined. Remove 2-1/2 cups batter from bowl and set aside. Beat in remaining 1 tablespoon milk and 2 tablespoons cocoa powder to remaining batter.
Spray two-tone cupcake insert with vegetable pan spray and place in prepared pan. Pipe peanut butter batter into outer ring, and pipe chocolate batter into inner ring. Repeat until baking cups are filled.
Bake 18-20 minutes or until toothpick inserted in center of cupcake comes out clean. Cool in pan on cooling grid 5 minutes; remove cupcakes from pan and cool completely on grid.
For topping, melt 1/3 cup peanut butter in microwave-safe bowl on HIGH 30 seconds or until melted and smooth. Dip tops of cupcakes in peanut butter; refrigerate 10 minutes or until peanut butter is firm.
For icing, beat shortening and 1/2 cup butter in large bowl with electric mixer until smooth. Add 3/4 cup cocoa and 1 teaspoon vanilla; beat until smooth. Gradually add confectioners’ sugar, 1 cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. Add 1/4 cup milk and beat until light and fluffy. Spread or pipe icing onto cupcakes. Melt remaining 1/3 cup peanut butter in microwave-safe bowl on HIGH 30 seconds or until melted and smooth; drizzle over cupcakes. Sprinkle with chopped peanuts.