1. Preheat oven to 350 degrees. Butter and flour an 8-inch round fluted tart pan with a removable bottom or a springform pan, tapping out excess.

2. In the bowl of an electric mixer fitted with the whisk attachment, whisk together egg whites and sugar until medium-stiff peaks form.

3. Meanwhile, melt chocolate and butter in a large heatproof bowl set over (but not touching) simmering water, stirring occasionally until smooth. Remove from heat.

4. Add 1 yolk to chocolate mixture and beat to combine. Add remaining yolks and mix to combine. Add confectioners' sugar, almond flour, and all-purpose flour; mix until combined. Add 1/3 of the egg white mixture and mix to loosen chocolate mixture. Gently fold in remaining egg white mixture.

5. Transfer to prepared cake pan and evenly spread. Transfer cake pan to oven and bake until puffed and knife inserted into the center comes out clean, about 17 minutes. Transfer pan to a wire rack and let cool 10 minutes. Invert cake onto wire rack. Carefully re-invert and let cool completely. Dust with cocoa powder, if desired.