Preheat oven to 350 F. Grease and flour two 9x1 1/2-inch round baking pans or one 9 x 13-inch pan; set aside.
In a large mixing bowl combine flour, sugar, Clabber Girl Baking Powder, baking soda, and salt. Add buttermilk, butter, and vanilla. Beat with an electric mixer on low to medium speed for 30 seconds or until combined. Beat on medium to high speed for 2 minutes, scraping sides of bowl occasionally. Add eggs and cooled chocolate, beat for 2 minutes more.
Pour batter into the prepared pans. Bake in a 350 F oven for 30 to 35 minutes or until a toothpick inserted near the centers comes out clean. Cool in the pans on wire racks for 10 minutes. Remove cakes from the pans. Cool completely on the racks.
Chocolate Sour Cream Frosting: In a small saucepan heat and stir 1 1/2 cups semisweet chocolate pieces and 3 tablespoons margarine or butter until melted. Stir in 1/2 cup dairy sour cream and 1 teaspoon vanilla. Gradually add 3 1/2 cups sifted powdered sugar, beating until smooth.
Frost with Chocolate Sour Cream Frosting. Garnish top with chocolate curls and triangles. Store in the refrigerator.