Preheat oven to 350 degrees. Lightly butter and flour a 9 by 5 by 3 inch pan and set aside. Sift together the flour, cocoa, baking powder, baking soda and salt; set aside. Beat butter and sugar together until light and fluffy. Add the eggs, one at a time, beating about 1 minute after each addition.
Add the sifted ingredients alternately with the sour cream and chocolate chunks, beginning and ending with the dry ingredients. (The batter will be quite thick.) Turn the batter into the prepared loaf pan and bake 1 hour to 1 hour and 15 minutes. To test, stick a toothpick in the center; bread is ready when toothpick comes out clean. Let cool in pan 15 minutes and remove. Makes one loaf.