Peel the chestnuts if that hasn't been done yet. Then add them to a blender and blend them into a dry and fine crumble. Put it aside until needed later.
Let the butter and chocolate melt. Whisk into a smooth mixture and let it cool down again. Then add the chestnut crumble from the blender, the plain flour and the egg yolk.
Stir well. Then stir in the cream.
Also add the beaten egg white to the chocolate batter.