Melt together the butter and chocolate over low heat until it combines into a smooth glossy mixture. Set aside to cool down to room temperature.
Separate egg yolks and whites.
Add a dash of cream of tartar and whip whites to half-stiff peaks.
Mix the rest of the ingredients on a bowl, add chocolate mixture, then add whipped egg whites and mix well.
Bake in 140°C oven (no fan), until just slightly jiggly in the middle, about 50-60 minutes.