Chocolate Buckwheat Cake

Ingredients

• 1 tsp. instant yeast
• ½ cup (63 g) buckwheat flour
• 1 Tbsp. honey, preferably buckwheat
• Nonstick vegetable oil spray
• 4 large eggs
• ¾ cup (135 g) bittersweet chocolate chips
• ½ cup (1 stick) unsalted butter, cut into pieces
• ¼ cup (25 g) Dutch-process unsweetened cocoa powder, sifted
• 1 Tbsp. vanilla extract
• Pinch of kosher salt
• ½ cup (100 g) granulated sugar, divided
• 2 Tbsp. powdered sugar
• A 9"-diameter springform pan

Steps

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. 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.
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.
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.