Dissolve cocoa in boiling water; cool. In a large bowl, cream butter and sugar until light and fluffy. Add eggs, 1 at a time, beating well after each. Beat in vanilla.
Combine the flour, baking soda, baking powder, and salt; gradually add to creamed mixture alternately with sour cream, beating well after each addition.
Add cocoa mixture and mix well.
Pour into 3 greased and floured 9-inch round baking pans. Bake at 350 degrees for 30-35 minutes or until a toothpick inserted near the center comes out clean.
Cool for 10 minutes before removing from pans to wire racks to cool completely.
In a microwave, melt chocolate chips and butter; stir until smooth. Cool for 5 minutes.
Transfer to a large bowl. Beat in sour cream and vanilla. Add confectioners' sugar; beat until light and fluffy.
Spread between layers and over top and sides of cake. Store in the refrigerator.