Heat oven to 375 degrees F. Line the cups in two standard-size 12-cup muffin pans with paper liners.
Chocolate Cupcakes: In medium-size bowl, combine flour, cocoa powder, baking soda and salt.
In large bowl, with mixer on medium-high speed, beat butter and sugar until smooth and creamy, 2 minutes. Beat in eggs and vanilla until fluffy, 1 minute. On low speed, beat in flour mixture alternately with milk, beginning and ending with flour mixture. Fill each prepared muffin cup two-thirds full, about 3 tablespoons.
Bake in 375 degrees F oven 20 minutes or until wooden pick inserted in centers comes out clean. Remove cupcakes from pan to wire rack; let cool.
Filling: In medium-size bowl, with mixer on medium speed, beat peanut butter and butter until smooth. On low speed, gradually beat in sugar and 3 tablespoons milk. On high speed, beat until light and fluffy, 1 minute. If too thick, beat in 1 tablespoon milk. When cupcakes are cooled, spoon filling into large pastry bag fitted with 1/4-inch plain round tip. Insert tip into top of each cupcake; squeeze as much filling into center of each as possible (top will bulge).
Chocolate Frosting: In large glass bowl, microwave milk chocolate pieces on high power 1 to 2 minutes, stirring to melt. With mixer on medium speed, beat in sour cream until fluffy and good spreading consistency. Frost cupcakes. Sprinkle edge with peanuts, if desired. Store, covered, in refrigerator up to 3 days.