Preheat the oven to 180°C.
Melt the chocolate and the butter in glass bowl above simmering water. Mix well and let cool a bit.
Meanwhile whisk the eggs the sugar until foamy, then add to the melted chocolate. Carefully fold in the flour.
Finely chop the chestnuts, then fold into the batter.
Line a square pan (about 22cm) with baking paper, then spoon the batter into it and spread out evenly.
Bake for about 20 minutes, until almost cooked through (the middle should remain a bit gooey).
When completely cooled, slice into cubes.
Dict with icing sugar or cocoa powder if you like.