Heat oven to 350°F. Coat two 8 x 2-in. round cake pans with nonstick spray. Line bottoms with wax paper; spray paper.
Make Cake: Pulse chocolate and the 1 1⁄2 cups sugar called for in batter in a food processor until chocolate is ground.
Beat butter, sugar, chocolate, baking powder and vanilla in a large bowl with mixer on high 3 minutes or until fluffy. Add eggs, 1 at a time, beating well after each. On low speed, beat in flour in 3 additions alternately with milk in 2 additions, just until blended, scraping sides of bowl as needed. Divide between prepared pans; spread evenly.
Bake 35 minutes or until a wooden pick inserted in center of cakes comes out clean. Cool in pans on a wire rack 10 minutes, invert on rack, remove paper and cool completely.
Make Chocolate Buttercream: Beat butter and chocolate in a large bowl with mixer on medium speed until creamy. Beat in Marshmallow Fluff. When blended, beat in confectioners’ sugar and vanilla. Increase speed to high; beat 3 to 4 minutes until fluffy.
Put 1 cake layer on a serving plate. Spread top with 1 cup frosting. Top with other cake layer. Frost sides, then top with remaining frosting.
