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Preheat the oven to 325F. Butter and lightly flour a 9x5x3-inch loaf pan.
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In the bowl of a stand mixer fitted with a paddle attachment, 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.
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Sift the flour, cocoa, baking 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.
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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.