Melt the butter. Peel the bananas, mash them with a fork and mix with the cream. Beat the eggs with the sugar until frothy. Slit the vanilla pod lengthways, scrape out the pulp and add to the egg mixture (vanilla sugar can also be used as an alternative).
Mix the flour with the ground nuts and the baking powder and stir into the egg mixture, tablespoon at a time, with the melted butter. Mix the banana puree into the batter.
Grease a bowl cake or wreath dish well and sprinkle with breadcrumbs. Pour in the dough and bake at 170 degrees for 45-60 minutes. Melt the couverture and coat the cake with it. Sprinkle with chopped pistachios if necessary.