1. Heat oven to 350 degrees F. Coat three 8-inch-round cake pans with cooking spray. Line bottoms with waxed paper rounds. Spray paper.
2. In large bowl, whisk flour, baking powder, baking soda and salt.
3. In second large bowl, beat butter and sugar on medium-high 3 to 4 minutes, until smooth and light; scrape down sides halfway through. Add eggs, one at a time, beating well after each. Add vanilla; beat to mix.
4. On low speed, beat in flour mixture in 3 additions, alternating with buttermilk and ending with flour. Scrape down sides of bowl; beat on medium 3 minutes. Fold in chocolate. Divide batter among pans, using scant 3 cups per pan; spread evenly in pans.
5. Bake in 350 degrees F oven 35 minutes, until cakes spring back when lightly touched. Let cakes cool in pans on wire racks 10 to 15 minutes. Turn out cakes; let cool. Remove waxed paper.Frosting:
6. In large bowl, beat butter on medium-high until smooth and creamy, 2 minutes. On low speed, beat in 1 box sugar and half the buttermilk until blended, 1 to 2 minutes; scrape down sides of bowl as needed. Beat in remaining box sugar, buttermilk and vanilla; beat until smooth, 1 to 2 minutes. Beat in melted chocolate. Cover bowl with plastic wrap (can be kept at cool room temperature up to 1 hour).Assembly:
7. Place one cake layer on cake stand or platter. Frost top with 1 cup frosting. Place another layer on top; frost top with 1 cup frosting. Top with third layer. Reserve 1 cup frosting for top. Frost side of cake with remaining frosting; then frost top with reserved 1 cup. Sprinkle with chopped chocolate. Refrigerate at least 1 hour before serving.