4 tablespoons (1/2 stick) unsalted butter, preferably cultured, plus more for the pan
1 tablespoon all purpose flour, plus more for the pan
3 large eggs, separated
1 teaspoon granulated sugar
3 ½ ounces bittersweet chocolate, preferably Valrhona 66% cacao, chopped
½ cup confectioners’ sugar
½ cup almond flour
Dutch process cocoa powder, optional
Preheat the oven to 350 degrees. Butter and flour an 8-inch round fluted tart pan with a removable bottom or a spring form pan, tapping out the excess.
Whisk the egg whites with the granulated sugar until medium-still peaks form.
Meanwhile, fill a medium saucepan with an inch of water and bring to a simmer. Combine the chocolate and butter in a heatproof bowl and set over the simmering water. Melt, stirring occasionally. Remove the bowl from the saucepan.
Beat 1 yolk into the chocolate mixture to temper it. Beat in the remaining yolks, then beat in the confectioners’ sugar, almond flour, and all-purpose flour until well mixed.
Add a third of the whipped whites and beat well to loosen chocolate mixture. Gently fold in the remaining whites just until combined. Pour into the prepared pan and gently spread evenly.
Bake until puffed and a knife comes out clean, about 17 minutes. Let cool in the pan on a wire rack foe 10 minutes, then invert onto the rack. Carefully flip the cake right side up. Let cool completely. Dust with cocoa powder if you’d like.