Marble cake with chocolate frosting

Preheat the oven to 350F and line two 9-inch pans with parchment paper.

Chop the bittersweet chocolate and melt it over a double boiler or in the microwave.

Sift the cake flour, baking powder and kosher salt, whisk and set aside.

Cream the butter in a stand mixer with the paddle until creamy, add the sugar and mix until light and fluffy, about 3 minutes; scrape the bowl down.

Add the eggs one at a time, letting each incorporate; scrape the bowl again.

Add the vanilla. With the mixer on low, alternate the flour mixture and the buttermilk in three batches, working quickly, and mix until just combined so the batter stays light.

Divide the batter in half: put half in the pans and reserve about 330 g (about one and a half cups); mix the melted chocolate and a quarter cup of buttermilk into the reserved batter.

Dust the sides of the buttered, lined pans with flour.

Add the vanilla and chocolate batters back and forth, then swirl with a spatula or knife, dragging the vanilla through the chocolate into ribbons without overmixing.

Wrap the pans in wet cake strips (optional) for even, flat layers and soft edges. Bake 30 to 35 minutes at 350F, until the centres are set and a skewer comes out clean.

Frosting: cream the room-temperature unsalted butter a few minutes, then sift in the cocoa powder with the salt and mix well, scraping the bowl, so no lumps of butter remain.

On low, add the confectioner's sugar a little at a time, alternating with the cream drizzled in, without beating in too much air. Add the optional vanilla; add another tablespoon of cream for a thinner consistency.

Cool the cakes in the pans about 20 minutes, then invert onto a cake plate; knock the pan on the side if a layer sticks.

Put a dollop of frosting under the first layer to hold it, spread about half a cup on top, invert the second layer over it, and frost the top and sides, swirling the finish.