Chop the banana and finely puree it with the chestnuts, cocoa powder, sugar and butter. I use the cup attachment with the integrated cutting knife of my hand blender for this.
Pour the chestnut chocolate spread into a clean 350 ml screw-top jar and store in the refrigerator. The spread lasts for about 14 days.
If you like, you can add 1 teaspoon of cinnamon to the spread.