Preheat the oven to 325°F.
Pour some water into a pot and place it on medium heat.
Chop the chocolate and add it to a saucepan together with the butter.
Place the saucepan in the pot.
Melt the chocolate stirring continuously.
Crush the banana with a fork and add the sugar. Stir to mix well.
Pour the chocolate mixture over the banana.
Separate the yolks from the whites.
Add the yolks, one by one, to the chocolate-banana mixture and stir to mix.
Set aside to cool down.
Whisk the whites until stiff peaks form.
Slowly add the beaten whites to the chocolate mixture.
Grease 6 souffle forms with butter and divide the mixture.
Bake for 20 minutes in the preheated oven.
Dust with powdered sugar and serve immediately.