Preheat the oven to 350°F. Lightly grease two 3 1/4" x 5 3/4" mini loaf pans, or one 9" x 5" loaf pan., Combine the flour, baking soda, salt, and cocoa., In a separate bowl, beat the butter and sugar until light and creamy., Beat in the egg, then stir in the vanilla, banana, and sour cream., Gently mix in the dry ingredients and chocolate chips until well incorporated., Pour the batter into the pan(s) and bake for 45 to 60 minutes (bake for the shorter time for two mini loaves and the longer time for the larger loaf), until a cake tester or toothpick inserted into the center comes out clean., Remove the bread(s) from the oven, and allow to rest in the pan(s) for 10 minutes. Turn the bread(s) out onto a rack to cool completely.