Combine the Dutch process chocolate with 2 tbsp sugar, the cinnamon and 4 tbsp warm milk to obtain a smooth mixture.
Beat the softened butter with 2/5 cup (100 g) sugar to a fluffy cream and add the yolks of the eggs 1 by 1. Flavor with the vanilla.
Separately, beat the egg whites with the remaining sugar.
To the mixture with the yolks, pour 1/3 of the egg whites, while alternating between the flour and milk. Continue pouring and stirring nonstop until the mixture homogenizes. Use a spatula, not a mixer.
Mix half of this mixture with the chocolaty mixture and pour it into an oiled and lightly floured cake form (8 1/2″ (22 cm) in diameter).
Distribute the remaining lighter part on top and put the cake to bake in a preheated 356°F (180 °C) oven with the fan on. Check to see if it's ready with a toothpick once the cake has fluffed up.