Preheat oven to 350 F. Grease 2 8-inch cake pans and line the bottoms with parchment paper.
Cream butter and brown sugar in the bowl of a stand mixer with a paddle attachment on medium speed until light and fluffy.
Add eggs, one at a time, beating well after each addition.Beat in vanilla and coffee.
Whisk together flour, cocoa, baking soda and salt in a medium bowl.
Add the flour mixture to the wet ingredients in thirds, alternating with sour cream, and beating on low speed after each addition.
Stir in water and coffee until blended.
Pour cake batter into prepared pans. Bake 30-35 minutes in the center of oven, or until a toothpick inserted in the center comes out clean.
Let the cake cool in pans 10 minutes before removing to a wire rack.
Freeze the cake for at least 1 hour, and then level the top of the cake and cut each of the cake layers in half. The cake can be quite crumbly and is much easier to work with when frozen.