Preheat oven to 350 degrees.
Grease and lightly flour four 8 or 9 inch round pans.
Set pans aside.
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.
Divide batter evenly between prepared pans.
Bake 20 - 30 mintues or until a wooden toothpick inserted near center comes out clean.
Cool cakes in pans on wire racks for 10 minutes.
Remove cakes from pans.
Cool layers thoroughly on wire racks.

Cookie Filling:
In large mixing bowl combine powdered sugar, butter, 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.

To assemble the cake, put a cake layer on a serving plate and top with 1/3 of the cookie filling, spreading evenly over layer.
Add a second layer of cake and spread with another 1/3 of the cookie filling.
Add another cake layer and spread with remaining filling.
Top with remaining cake layer.
Spread top and sides of cake with frosting.
Stir reserved cookie filling and enough milk 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.

Chocolate Sour Cream Frosting:
In a saucepan melt 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.
Cover and store in the refrigerator.