To make the cake: Grease and flour two 9 by 1 1/2-inch round baking pans; set aside. Preheat oven to 350 degrees F.
In a large mixing bowl, combine flour, sugar, baking powder, baking soda, and salt. Stir to combine thoroughly. Add buttermilk, shortening 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 degree F. oven for 30 to 35 minutes or until a toothpick inserted near the center comes out clean. Cool in the pans on wire racks for 10 minutes. Remove cakes from the pans. Cool completely on the racks.
To make the frosting: In a small saucepan heat and stir 1 1/2 cups semisweet chocolate pieces and 3 Tbsp. butter until melted. Reserve 1 Tbsp. to drizzle on cake. Stir in 1/2 c. of sour cream and 1 tsp. vanilla. Gradually add 3 1/2 c. sifted powdered sugar, beating until smooth. Makes about 2 cups.
To put it all together: Frost the cake once it is completely cooled. Use some of the reserved chocolate/butter mixture to drizzle over the top. Store in the refridgerator. Makes 12 servings.