In a bowl, beat 1 cup butter, 1 1/3 cups sugar, cocoa, and chocolate with a mixer until fluffy. Add eggs and sour cream. Beat until well mixed.
Mix together soda, baking powder, and 2 cups flour. Stir into batter, then beat to blend well.
Butter and flour-dust a 10- to 12-cup tube pan. Scrape batter into pan and spread level. Bake in a 325° oven until cake just begins to pull from pan sides and center springs back when lightly pressed, about 1 hour.
Meanwhile, in a 1 1/2- to 2-quart pan over high heat, stir 2 cups sugar with 1 cup water until sugar dissolves. Remove syrup from heat and add liqueur.
Run a knife blade between cake and pan. Let cake cool in pan on a rack for about 10 minutes; invert cake onto rack, then tip cake back into pan. With a thin skewer, pierce cake to pan, making holes an inch apart. Slowly pour syrup over cake. Let cool about 1 1/2 hours.
Dip cake pan in hot water almost to rim; wipe pan dry. Invert cake onto a plate.