Flour - 180 grams (6.5 oz)
Cocoa powder - 40 grams (1.5 oz)
#Butter - 70 grams (2.5 oz)
Sugar - 220 grams (8 oz)
Salt - a pinch
Vanilla sugar - 2 teaspoons
Sift 180 grams (6.5 oz) of flour and 40 grams (1.5 oz) of cocoa powder into a bowl.
Place 4 eggs, 4 egg yolks and 220 grams (8 oz) of sugar in a bowl, and set over simmering water. Whisking constantly, bring the egg mixture to 43 ° C (110 ° F).
Transfer to the bowl of a stand mixer; add a pinch of salt and two teaspoons of vanilla sugar. Whip on high speed until increased in volume and stiff.
Add the sifted flour and cocoa powder into the egg mixture, in 3 batches, folding gently after each addition.
Mix some of the cake batter with 70 grams (2.5 oz) of melted butter, and fold into the cake batter until incorporated.
Transfer to a 26 cm (10 inch) cake pan, lined with parchment paper and brushed with butter.
Bake at 190 ° C (375 ° F) for about 30 minutes. Test for doneness - an inserted skewer should come out clean.
Cool slightly, invert onto a rack, unmold, and cool completely for a few hours.