Heat oven to 325°F. Line 12 regular-size muffin cups with paper or foil liners.
Make Frosting: Beat ingredients on low speed to blend, then on high speed 2 minutes until light and fluffy. Refrigerate to firm up frosting.
Make Cupcakes: Melt butter and chocolate in a medium saucepan over low heat, stirring until melted and smooth. Remove from heat. Whisk in sugar and vanilla, then eggs, 1 at a time, until well blended. Stir in flour, cocoa and baking powder just to blend. Pour into muffin cups.
Bake 33 minutes, or until a wooden pick inserted in centers comes out with moist crumbs. Cool in pan on a wire rack (cupcakes may sink slightly).
Spoon frosting into a large ziptop bag, snip 1⁄2 in. off 1 corner and pipe swirls on top of cupcakes. Garnish each with a raspberry.
