Preheat the oven to 350°F (180°C) and grease a loaf pan. Set aside.
Melt butter and chocolate in a water bath or in the microwave, 10 seconds at a time, stirring in between each time.
Pour in the chestnut spread and add the eggs, one at a time, stirring vigorously between each increment to prevent raw eggs from cooking when in contact with warm melted chocolate. Add one tablespoon of flour, and stir well.
Pour the batter into the prepared loaf pan and bake for 45 minutes. Let cool completely before serving. Enjoy with vanilla custard (crème anglaise).