Grease a 10×15 inch (4-4.5 quart) glass baking dish with non-stick cooking spray.
In a large mixing bowl, cream the butter and brown sugar together until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla extract.
Sift the flour, cocoa powder, baking soda, and salt into another large mixing bowl. Whisk to combine. Add the wet mixture and sour cream to the dry ingredients and beat with an electric mixer just until well blended. Add the boiling water then stir until combined.
Pour the batter evenly into the prepared baking dish. Bake at 350 degrees for 45 minutes until a toothpick or knife is inserted into the center of the cake and comes out clean. Set the cake aside to cool.
Microwave the butter in a microwave safe bowl just until melted. Stir in the semi-sweet and unsweetened chocolate. Microwave in 30 second intervals, stirring between each interval, until the chocolate is melted.
In a large mixing bowl, combine the melted chocolate and butter mixture, powdered sugar, sour cream, and vanilla; beat with an electric mixer until smooth.
Spread the frosting evenly onto the cooled to room temperature cake.
Makes 18 servings. Leftover cake should be covered tightly and refrigerated.