Cupcakes: Stir together flour, pudding mix, sugar, baking powder, salt, and Oreos. Whisk together eggs, milk, oil, and vanilla. Stir into dry ingredients till just combined.
Fill greased or paper lined muffin cups 2/3 full. Bake at 350° for 18-20 minutes. Cool completely and frost with chocolate frosting.
Frosting: Beat butter and cocoa till smooth. Add remaining ingredients, using enough milk to get the consistency you like. Pipe onto the cupcakes using a large star tip. Sprinkle crushed Oreos on top if desired.