Melt semisweet chocolate morsels in a microwave-safe bowl at HIGH for 30-second intervals until melted (about 1 1/2 minutes total time). Stir until smooth.
Beat butter and brown sugar at medium speed with an electric mixer, beating about 5 minutes or until well blended. Add eggs, 1 at a time, beating just until blended after each addition. Add melted chocolate, beating just until blended.
Sift together flour, baking soda, and salt. Gradually add to chocolate mixture alternately with sour cream, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Gradually add 1 cup hot water in a slow, steady stream, beating at low speed just until blended. Stir in vanilla. Use immediately, following directions for desired cake.
The addition of hot water at the end of this recipe makes for an exceptionally moist cake.