Blanch the pumpkin in boiling water until soft, allow to drain. Transfer the cooked pumpkin to a blender, along with the banana. Add the cinnamon, ginger, nutmeg, sugar and vanilla sugar. Blend until smooth. Add enough milk to obtain the right consistency and blend again. Whip the cream, together with the icing sugar. Pour the milkshake into tall glasses. Top with the whipped cream and sprinkle with a bit of cinnamon.