Butter and flour two 9-inch round cake pans and preheat oven to 350 degrees.

In mixing bowl at medium speed, beat butter and brown sugar until blended.

Beat in eggs.

Add melted chocolate and vanilla; beat until combined.

In another bowl, stir together flour, cocoa, baking powder, baking soda, and salt.

In another small bowl, mix milk and sour cream.

Add flour mixture and milk mixture alternately to butter mixture, about a third of each at a time, beating after each addition.

(Batter will be very thick).

Spread batter evenly into cake pans.

Bake in 350 degree oven until a wooden skewer or toothpick inserted in the center comes out clean (25 to 30 minutes).

Cool on racks in pans for 10 minutes, then invert cakes onto racks and remove pans.

Cool completely before frosting.