Soften the chocolate in a double boiler. Add the butter, and as soon as it is very soft, remove from the heat and stir until smooth with a spatula.

Preheat the oven to 450°F (230°C). Separate the eggs. Place the 4 yolks in a mixing bowl with the granulated sugar. Whisk thoroughly until the mixture becomes thick and pale, like a sabayon.

Place the 5 egg whites in a separate bowl and beat until stiff peaks form. Gradually add the powdered sugar while beating, until the mixture is glossy and forms a meringue.

Gently fold the chocolate mixture into the yolk mixture, then carefully fold in the meringue.

Divide the mixture among 4 bowls and bake for 4-5 minutes, until the tops start to crack and dome. Serve immediately.