1. Heat oven to 350 degrees. Line 18 indents in 2 cupcake pans with paper or foil liners. In a bowl, whisk together flour, cocoa powder, baking powder and salt.
2. Beat butter and sugar in a large bowl 2 minutes, till light colored and smooth. Add eggs, one at a time, beating well after each. On low speed, add flour mixture, alternately with milk. Divide batter among liners, a scant 1/3 cup in each.
3. Bake at 350 degrees for 19 minutes or until toothpick inserted in center emerges clean. Remove to a rack; cool completely.
4. Meanwhile, make Frosting. Combine chocolate and butter in a medium saucepan (not nonstick) over medium heat, stirring until melted, about 4 minutes. Beat in corn syrup. Add half the confectioners' sugar and beat until blended (mixture will look a little grainy). Beat in milk, followed by remaining confectioners' sugar and vanilla extract and beat until smooth, adding extra 1/2 cup confectioners' sugar if needed. Refrigerate for 15 minutes or until good spreading consistency. Spoon frosting into a piping bag fitted with a large star tip and pipe in swirled pattern over cupcakes.