Makes one (20-cm) square and 8cm deep cake tin, about 24 slices
- 2 2/3 cups all-purpose flour (spoon flour into a dry-measure cup and level off)
- 1 2/3 cups sugar
- 2 teaspoons baking powder
- 1/4 teaspoon salt
- 12 ounces/350 grams (3 sticks) unsalted butter, softened
- 7 large eggs
- 2 tablespoons dark rum
- 2 tablespoons milk
- 1/2 teaspoon baking soda
- 6 ounces (175 grams) bittersweet (not unsweetened) chocolate, melted and cooled
- 2 cups Base Batter
1 x 20cm square cake tin greased and lined with baking parchment.
Set a rack in the lower third of your oven and heat the oven to 160C/325F.
Combine flour,sugar,baking powder and salt, in the food processor.Pulse quickly to mix.Add the butter and beat on alow speed until you have a smooth heavy paste( 1-2 minutes).
Whisk the eggs together then on medium speed introduce them gradually into the flour and butter mix, beat for 1 minute,scraping down the beater bowl in between each introduction of the egg mixture.When all the egg mixture has been added beat for a further 2 minutes.Remove the bowl and set aside.
Combine the milk and baking soda in a medium mixing bowl.Whisk well to dissolve the soda. Scrape in the chocolate and whisk well. Add 2 cups of the base batter to the chocolate mixture and whisk again to combine. Scrape half the remaining base batter into the prepared tin and smooth the surface.Cover with the chocolate batter making another layer as even as possible.Top with the remaining base batter and smooth the top.Using a wide bladed table knife or metal spatula 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.