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1
Preheat the oven to 350F.
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2
Brush the interior of a 9 x 5-inch loaf pan with the melted butter, then sprinkle the greased surface with 1 generous tablespoon of cake flour.
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3
Tilt the pan to coat it with flour, then knock out any excess.
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4
Whisk together the remaining 1 2/3 cups of cake flour with the dried buttermilk, baking powder, and salt and sift this into another bowl.
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5
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 until smooth.
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6
Using an electric mixer fitted with the paddle attachment, or a handheld mixer, cream the softened butter until lightened.
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7
Add the sugar in increments, beating well after each addition.
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8
In a 2-cup liquid measuring cup (or other vessel with a spout), beat the eggs with the vanilla and zest, if using.
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9
Drizzle the eggs into the creamed butter mixture a little at a time, allowing each addition to be fully incorporated before adding the next.
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10
When done, the mixture should be very light.
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11
Stop the machine and, using a large rubber spatula, scrape the butter down from the sides and up from the bottom.
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12
Beat briefly.
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13
Stop the machine and add a heaping 1/2 cup of the flour mixture.
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14
Turn the machine to low, then up to medium and mix until the flour is almost totally incorporated.
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15
Continue to stop the machine, add another scoop of the flour mixture, and beat again until almost combined.
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16
After adding all the dry ingredients, beat the batter, still on medium speed, for 30 to 40 seconds, or until very smooth and silky looking.
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17
Scrape half the batter into another bowl and add the melted chocolate to the first bowl, folding it in until homogenous.
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18
Pour and spread half of the plain cake batter into the prepared loaf pan, then do the same with half of the chocolate batter.
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19
Repeat this with the plain batter and then the rest of the chocolate batter, creating four layers in all.
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20
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 loops at the top and then at the bottom while traveling to the opposite end of the loaf pan.
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21
(Dont overdo this, however, or youll actually lose your marbled design.)
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22
When done, pull the knife blade straight up and out of the batter, and gently smooth the top using a short metal spreader.
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23
Place the pan in the center of the oven and reduce the temperature to 325F.
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24
Bake for 1 hour and 10 minutes, or until a tester comes out clean and the cake is golden.
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25
Place the pan on a wire rack and carefully run a knife around the sides of the pan.
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26
Turn the cake out of the pan and let it cool on the rack.
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27
Just before serving, dust the top with confectioners sugar, if desired, and cut into thick slices.
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28
Plain Pound Cake
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29
Prepare the batter as directed, omitting the chocolate.
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30
Add the entire batter to the prepared pan and smooth the top.
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31
Bake for 65 minutes (about 5 minutes less than you would a marbled pound cake).
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32
Cool and serve as previously described.
