Black-and-white marble cake

My primary souvenir from this trip to NYC was Peyton and Byrne British Baking (plus a Strand tote bag to put it in). There are any number of exciting recipes in there that I look forward to sharing with you, like Sherry Trifle and Chelsea Buns and Victoria Sponge, but the most exciting ones were several levels above my baking capacity when I made it a few days ago. (How does a "non-stop" flight from Newark to SFO mean sitting on a plane for 9.5 hours and getting home at 3am west coast time? Hint: first answer the question "How many Continental airplanes does it take to change a lightbulb in Indianapolis?" Sigh and yawn.)

But a chocolate and vanilla marble cake, that seemed both festive enough for Chris's qual and simple enough for my jetlagged self. It doesn't have much to do with black-and-white cookies besides the vague NYC connection; maybe I should call it black-and-white-and-Brit instead. This is a 1.5x batch to upgrade it from loaf pan to bundt pan, plus some extra white chocolate in the cake, plus two ganaches instead of one frosting, plus some error bars from mass-to-volume conversions.

cake:
180 g (1 stick + 6 T) butter, softened
300 g (1.5 c) sugar
4 eggs
1.5 t vanilla
250 g (2 c) flour
2 t baking powder
1/2 t baking soda
250 g (just over 1 c) plain yogurt
100 g white chocolate, chopped
2/3 c cocoa powder
6 T boiling water

Preheat oven to 350F and butter and flour a bundt pan. Cream together the butter and sugar in a large mixing bowl, then beat in the eggs one at a time and the vanilla. Mix the dry ingredients (flour, bakings powder and soda) in a small bowl and add in two or three additions to the batter, alternating with the yogurt.

Dump about 1/3 of the batter into a medium bowl, and stir the white chocolate into the larger fraction. In yet another bowl, stir together the cocoa powder and water until thick and smooth, then stir the chocolate into the smaller fraction of the batter (using the electric mixer if needed to make it smooth).

Drop the batter into the bundt pan, alternating spoonfuls of the black and white batters. I put 5 or so spoonfuls of the white batter evenly spaced around the pan, then added spoonfuls of the black batter in the spaces, then covered the black batter with spoonfuls of the white batter, then put spoonfuls of the black batter over the first round's spoonfuls of the white batter, and then you probably get the point. When all the batter is in the pan, run a spatula or knife once around the pan through the center of the cake to marble it.

Bake for 50 min until a toothpick comes out clean with maybe a couple moist crumbs and the top is brightly golden. Cool in the pan for 15 min before carefully inverting onto a baking rack to cool completely (mine stuck to the bottom a bit). When cool, cover the cake and cooling rack together with plastic wrap and let sit overnight.

ganache:
7 oz dark chocolate, chopped
3.5 oz white chocolate, chopped
1 c cream

In the morning, make black and white ganaches by putting each chopped chocolate in a bowl, heating the cream just to a boil (3/4 c for dark chocolate, 1/4 c for white), pouring the cream over each chocolate, and stirring each until smooth. Unwrap the plastic wrap from the top of the cake but leave it underneath to catch the ganache drippings. When the dark chocolate has firmed up enough to make a good glaze, pour over the cake, letting it drip down the sides. When the dark chocolate has set a bit and the white chocolate is starting to firm up, drizzle the white chocolate over the cake too.