Chocolate Buckwheat Cake

Preparation

1. Sponge

   Step 1

   Place ¾ cup lukewarm (98°–105°) water in a medium bowl and sprinkle yeast over. Whisk in buckwheat flour and honey. Cover with a kitchen towel or plate. Let sit at room temperature until surface of sponge looks frothy with big and small bubbles across the top, about 1 hour.

2. Cake

   Step 2

   Place a rack in middle of oven; preheat to 350°. Coat pan with nonstick spray. Separate yolks from eggs over a large bowl to catch egg whites. Place yolks in another small bowl; set egg whites aside. Combine chocolate and butter in a medium heatproof bowl and set over a medium saucepan of simmering water (do not let bowl touch water). Melt chocolate, stirring occasionally, until smooth. Remove from heat and whisk in cocoa powder, vanilla, salt, and ¼ cup (50 g) granulated sugar. Add egg yolks one at a time, whisking until smooth after each addition. Stir in sponge.

   Step 3

   Add remaining ¼ cup (50 g) granulated sugar to bowl with reserved egg whites. Using an electric mixer on medium-high speed, beat until soft peaks form, about 4 minutes. Fold half of egg whites into batter until smooth, then fold in remaining egg whites until just combined. Scrape batter into prepared pan and smooth surface.

   Step 4

   Bake cake, rotating pan halfway through, until a tester inserted into the center comes out clean, 40–45 minutes. Let cake cool in pan. Run a paring knife around edges of cake, then unmold and place on a plate. Dust with powdered sugar just before serving.

   Do ahead: Cake (without powdered sugar) can be made 1 day ahead. Store tightly wrapped at room temperature.