Preheat the oven to 350ºF and butter two 8½ x 4½-inch loaf pans.
Combine the flour, cocoa powder, baking powder and baking soda in a mixing bowl and stir well. Set aside.
Beat the butter and sugar together in a stand mixer set at medium speed. Beat in the eggs, one at a time, continuing to beat the batter smooth after each addition.
Scrape the bowl, lower the mixer speed then add the sour cream and the dry ingredients in intervals until fully incorporated.
Scrape the batter into the prepared loaf pans and bake the cakes for 30-35 minutes, or until they are well risen and a toothpick, when inserted, comes out clean.
Cool the pans on a rack for 5 minutes then turn the cakes out of the pan.
Dust with powdered sugar, slice and serve.
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