In a large bowl, melt butter and chocolate in the microwave, then allow the mixture to cool.
100 g butter, 250 g dark chocolate
Whisk in the powdered sugar and then the eggs, one by one, mixing well.
120 g powdered sugar, 4 eggs
When the batter is smooth, line a 9.5 x 5 bread loaf pan with parchment paper, (or grease it with butter and dust it with flour) and fill it by 2/3 with the batter.
Bake the cake it the preheated oven for about 25 minutes. Serve warm or at room temperature. The cake keeps well in the fridge for a few days, but won't last that long.