Heat the oven at 150°C. Whip the butter, sugar and eggs. Add the flour and baking powder and stick with a spatula. In the microwave (2 times 15 seconds) or in a bain-marie, melt the chocolate.(the quicker,the better). Put the paste in 4 bowls and mix the chocolate in 2 of this bowls.
In a cake tin, put a coat of paste, then a coat of chocolate, then paste ans to finish chocolate. Cook for about an hour at 150°C and check it's well-cooked in the middle (with a knife or a needle).