Step 1

Set a rack in the lower third of the oven and preheat to 325°F (160°C).

Step 2

In the bowl of an electric mixer, combine the flour, sugar, baking powder, and salt. Stir well by hand to mix. Add the butter. Beat the mixture on low speed with the paddle until the mixture is a smooth, heavy paste, 1 to 2 minutes.

Step 3

Whisk the eggs and rum together. On medium speed, beat 1/3 of the egg mixture into the flour and butter mixture. Beat for 1 minute.

Step 4

Stop and scrape down the bowl and beater. Add half of the egg mixture and beat for 2 minutes. Repeat with the other half.

Step 5

Remove the bowl from the mixer and using a large rubber spatula give the batter a final mix.

Step 6

For the chocolate batter, combine the rum, milk, and baking soda in a medium mixing bowl, Whisk well to dissolve the baking soda. Scrape in the chocolate and whisk it well. Add the 2 cups of base batter to the chocolate mixture and whisk well to combine.

Step 7

Scrape half the remaining base batter into the prepared pan and smooth the top. Cover with the chocolate batter, making it as even a layer as possible. Finally top with the remaining base batter and smooth the top. Use a wide-bladed table knife or a thin metal spatula to marble the batter: Insert the knife into the batter at the central tube, with the flat side of the blade facing you. Draw the blade through the batter to the bottom of the pan and up and out of the side of the pan closest to you, repeating the motion every inch or so around the pan, making a spiral in the batter, almost as though you were folding egg whites into it. Stop when you get back to the point where you started. Don't bother to smooth the top of the batter—it might disturb the marbling.

Step 8

Bake the cake until it is well risen and firm, and a toothpick or a small thin knife inserted midway between the side of the pan and the central tube emerges dry, about 1 hour.

Step 9

Cool the cake in the pan for 5 minutes, then invert a rack over it. Invert and lift off the pan. Cool the cake completely over the rack.