For the banana coconut soufflé, first cream butter until fluffy, stir in egg yolks. Add coconut flakes and vanilla sugar. Finely mash bananas with a fork and stir into the mixture. Beat egg whites with sugar until stiff and carefully fold into the mixture.
Butter soufflé molds and sprinkle with sugar. Pour 2/3 of the mixture into the molds. Cover molds with heat-resistant plastic wrap and place in steamer on a rack (or perforated cooking tray). Bake the banana-coconut soufflé at 90 °C for approx. 35 minutes.