Preheat the oven to 190°C/375F and line a cake tin with.
Heat a saucepan with water 2 inches high on medium heat and place the chocolate and butter into a bowl and sit on top of the simmering water.
Stir the chocolate and butter until smooth and no lumps remain.
Set aside to cool.
In a bowl, whip the egg whites until stiff, approximately 3-4 minutes with a hand beater.
Add the sugar a tablespoonful at a time while continuing to beat the egg whites, incorporating for 20-30 seconds until the sugar has dissolved.
Add the egg yolks into the fluffy egg whites and beat on medium until well combined, approximately 1 minute with a hand beater.
When the bowl of the chocolate feels cool to the touch, pour into the egg mixture and fold until well incorporated.
Pour into your prepared cake tin and bake for approximately 50 minutes until the cake has risen and a skewer comes out clean in the center.
Remove and allow to cool, the cake will fall in the center within 5 minutes of being removed from the oven.
Allow to cool in the tin and then place on a serving plate.
Sprinkle a little powdered sugar on top.
Slice and serve with a dollop of thick Greek yogurt or cup of milk.
Dig in!