Preheat the oven to 190c°C and line a 20 cm square baking tin with parchment paper.
Melt the chocolate with the butter in a glass bowl over simmering water.
In a large mixing bowl, whisk the egg yolks with the sugar until frothy. Stir in the chocolate mixture, then the baking powder.
Whisk the egg whites until they form stiff peaks, then fold it into the chocolate mixture.
Transfer the batter into the tin, then bake for 20-25 minutes.
Let it cool slightly in the tin then move to a wire rack.
When cooled, dust withy icing sugar.
Serve with ice cream, custard or vanilla yoghurt.