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Preheat the oven to 350ºF. Brush the interior of a 9 x 5-inch loaf pan with the melted butter, and then sprinkle the greased surface with 1 generous tablespoon of cake flour. Tilt the pan to coat it with flour, and then knock out any excess.
Whisk together 1 2/3 cups of cake flour with the dried buttermilk, baking powder, and salt and sift this into another bowl. Melt the chocolate either in the top of a double boiler over barely simmering water, or in a microwave, on high power, for 1 minute (stirring after the first 30 seconds). Using an electric mixer, fitted with the paddle attachment, beat the softened butter, until lightened. Add the sugar, in increments, beating well after each addition. In a 2-cup capacity liquid measuring cup (or other vessel with a spout), beat the eggs with the vanilla and zest, if using. Drizzle the beaten eggs into the creamed butter mixture, a little at a time, allowing each addition to be fully incorporated before adding the next. When done, the mixture should be very light. Stop the machine and, using a large rubber spatula, scrape the butter down from the sides and up from the bottom. Beat briefly. Stop the machine and add a heaping 1/2 cup of the flour mixture. Turn the machine on low, then up to medium and mix until the flour is almost totally incorporated. Continue to stop the machine, as you add another scoop of the flour mixture, and then beat again until almost combined. After adding all the dry ingredients, beat the batter, still on medium speed, for 30 to 40 seconds, or until the batter is very smooth and silky looking.
Scrape half the plain batter into another bowl and add the melted chocolate to the first bowl, folding it in until homogenous. Use a large, clean rubber spatula to transfer half the plain cake batter to the prepared loaf pan and, using a short, metal, off-set spreading spatula, spread batter out, covering the bottom of the pan. Do this same thing with half of the chocolate batter, using a clean off-set spatula, spreading it on top of the first layer of plain batter. Repeat this with the plain batter and then with the rest of the chocolate batter, creating 4 layers in all.
Beginning at one end of the loaf, drag a plain table knife through the batter, going up and down, in a soft sweeping swirling motion, making large loops at the top and then at the bottom, while traveling to the opposite end of the loaf pan (think of making a figure-8 over and over again). Don’t overdo this, however, or you’ll actually lose your marbled design. When done, pull the knife blade straight up and out of the batter, and gently smooth the top, using a short metal spreader. Place the pan into the center of the preheated oven and reduce the temperature to 325oF. Bake for 1 hour and 10 minutes, or until a tester comes out clean and the cake is golden. Place the pan on a wire rack and, carefully run a knife around the sides of the pan. Turn the cake out of the pan and let it cool on the rack. Just before serving, dust the top with confectioner’s sugar, if desired, and cut into thick slices.
Prepare the batter as directed, omitting the chocolate. Add the entire batter to the prepared pan and smooth the top. Bake for 65 minutes (about 5 minutes less than you would a marbled pound cake). Cool and serve as previously described.
If you love pound cake (and who doesn’t?) why not keep several sealed bags of the dry mixture in your pantry? That way, it’s much quicker to get this cake into the oven.