Heat your oven to 325°F. Butter and lightly flour a standard loaf pan. In a large bowl, on the medium speed of an electric mixer, cream the butter until smooth. Add the sugars and beat until fluffy, about 3 minutes. Add the egg and beat well, then the buttermilk and vanilla. Don’t worry if the batter looks a little uneven. Sift the flour, cocoa, bicarbonate of soda, baking powder and salt together right into your wet ingredients. Stir together with a spoon until well-blended but do not overmix. Scrape down the batter in the bowl, making sure the ingredients are well blended.
Pour the batter into the prepared loaf pan. Bake for 60 to 70 minutes, or until a cake tester inserted into the center of the loaf comes out clean. Cool in pan on a rack for about 10 to 15 minutes, at which point you can cool it the rest of the way out of the pan.