Preheat the oven to 160° conventional heat. Butter a 20cm round (without hinge) tin - it must be high enough to hold the dough, mine is 7cm high. Place a round of baking paper in the bottom. Flour the sides then tap to remove the excess.
Melt the butter in the microwave or in a saucepan, let it cool.
Whisk the egg yolks with the powdered sugar, brown sugar and vanilla. The mixture should turn white, this takes about 2-3 minutes (I use a mixer).
Pour in the warmed melted butter and mix.
Sift together the flour, bitter cocoa and salt. Incorporate these dry ingredients into the dough. Mix just enough to no longer see any powder (do not insist).
Warm the milk/cream mixture. Add these liquids to the dough in 3 batches, mixing well between each addition to maintain a uniform consistency. The dough becomes very liquid and this is normal.
Whip the egg whites until stiff and add them to the dough in 3 batches. Use a whisk to roughly break them up while incorporating them: you don't want a homogeneous mixture, the whites remain on the surface, a little thick.
Pour the batter into the prepared pan. Bake for around 50 minutes (this varies from one oven to another). The sponge cake rises a little but goes back down after cooking. The center should be a little wobbly when you move the mold, a sign that it will be creamy for tasting.
Leave to cool on a rack then place in the refrigerator for at least 3 hours. Refrigeration is what allows the magic cake to hold together well when cut.
To unmold it, pass a knife blade all around, and tap the mold on the work surface to release it easily. Sprinkle with a little bitter cocoa before serving.
Notes
Once cooked, the magical chocolate cake keeps for 3-4 days in the refrigerator : this will allow it to gain a nice consistency, and we will be able to cut it more easily.