Break the chocolate bar into pieces, add the butter in pieces to a small saucepan, and melt over a double boiler.
Separate the eggs. Pour the yolks into a mixing bowl along with 100g of sugar.
Beat the egg yolk and sugar mixture with a whisk until pale.
The chocolate and butter mixture should now be completely melted and smooth.
Add the yellow batter and sugar, and mix with a wooden spoon. Add the flour (previously sifted with the baking powder), and mix with a wooden spoon. Add the milk to thin the batter and make it easier to fold in the egg whites, then mix well.
Place the egg whites in a large mixing bowl with a pinch of salt to loosen them up. Begin whisking the egg whites (to "break" them up). Add 20g of granulated sugar. Continue whisking until the egg whites are stiff. Check the consistency.
Gently fold the beaten egg whites into the mixture in several batches.
Pour into a silicone mold that is 20 cm in diameter and at least 4 cm deep.
Preheat the oven to 170°C (top and bottom heat). Place on the middle rack.
Bake for 45 minutes at 170°C, with heat from both top and bottom.
Notes
A few suggestions for those with a sweet tooth:
1) Cut the cake (once cooled) in half and fill with ganache 2) Top with whipped cream 3) = 1+2!