In an extra-large mixing bowl combine flour, sugar, baking soda, salt, and baking powder. Add milk, sour cream, melted chocolate, butter, eggs, and vanilla. Beat with an electric mixer on low speed about 1 minute or until mixture is combined. Beat on high speed 3 minutes, scraping sides of bowl occasionally.
Place half of the batter in a medium bowl; cover and chill. Divide remaining half of batter evenly between the prepared pans. Bake in a 350 degree F oven for 20 to 25 minutes for 9-inch layers and 25 to 30 minutes for 8-inch layers or until a wooden toothpick inserted near centers comes out clean. Cool cakes in pans on wire racks for 10 minutes. Remove cakes from pans. Cool layers thoroughly on wire racks.
Wash the 2 cake pans. Grease and lightly flour pans. Divide reserved, chilled batter evenly between pans. Bake and cool as directed above.
Prepare Cookie Filling, reserving the 3/4 cup filling (without crumbs) for the cake garnish.
To assemble the cake, put a cake layer on a serving plate and top with one-third of the remaining cookie filling, spreading filling evenly over layer. Add a second layer of cake and spread with another one-third portion of the cookie filling. Add another cake layer and spread with remaining cookie filling. Top with remaining cake layer.
Prepare the Chocolate-Sour Cream Frosting. Spread top and sides of cake with frosting. Stir reserved Cookie Filling and enough milk (about 2 to 3 teaspoons) to make it a drizzling consistency. Spoon onto center top of cake. Carefully spread filling to force some down sides of cake. Garnish top of cake with the 4 crushed chocolate sandwich cookies. Cover and chill to store. Makes 16 servings.
In large mixing bowl combine powdered sugar, butter, 2 tablespoons milk, and vanilla. Beat with electric mixer on low speed until combined; beat on medium speed until very smooth. Beat in enough additional milk until frosting is easy to spread. Set aside 3/4 cup of mixture. Crush 10 chocolate sandwich cookies; stir into remaining mixture.
Chocolate-Sour Cream Frosting
In a saucepan melt semisweet chocolate pieces and butter over low heat, stirring frequently. Cool about 5 minutes. Stir in sour cream. Gradually add powdered sugar, beating until smooth and easy to spread.