Flourless Chocolate-Orange Cake

Place a rack in middle of oven and preheat to 350°. Butter bottom and sides of pan with unsalted butter. Remove a wide strip of orange zest from 1 large orange with a peeler and set aside. Finely grate remaining zest from orange into a large bowl; add 1 cup (200 g) sugar. Using your fingers, massage zest into sugar until mixture is homogenous and fragrant, about 1 minute. Set orange sugar aside.

Cut orange in half and squeeze 2 Tbsp. juice into a medium saucepan. Add 8 oz. bittersweet or semisweet chocolate, chopped, and 10 Tbsp. unsalted butter, cut into pieces. Place over medium heat and cook, stirring constantly with a heatproof rubber spatula and scraping down sides as needed, until melted and combined, about 2 minutes. Remove from heat and whisk in ½ cup plus 1 Tbsp. (45 g) unsweetened cocoa powder, 1 tsp. baking powder, and ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt. Let chocolate mixture cool slightly.

Add 3 large eggs to reserved orange sugar and whisk to combine (you can use the same whisk without cleaning as it’s all going to the same place). Scrape in chocolate mixture and whisk to combine. Scrape batter into prepared pan and bake cake until risen and top is set, 28–30 minutes. Transfer pan to a wire rack and let cake cool in pan. Remove sides of pan.

Just before serving, whisk 1 cup heavy cream in a medium bowl to form soft peaks; mound on top of cake, creating decorative swooshes with spoon. Using a sharp knife, very thinly slice reserved orange zest strip. Scatter orange zest and shaved bittersweet or semisweet chocolate over cake. Do Ahead: Cake can be baked 1 day ahead. Tightly wrap cooled cake and chill. Bring to room temperature before topping with whipped cream, orange zest, and shaved chocolate.