The white miso adds a wonderful salty, umami flavour which works so well in a cookie with chocolate (heaven!) but do use white miso instead of red as the red tends to be too strong.
Make the dough in advance, chill, roll into balls and open freeze. Once frozen put into bags and pull out and bake from frozen when needed. You will need to add a few minutes to the baking time if baking from frozen. You can also chill the dough, roll it into a sausage, wrap tightly in cling, freeze and then take out and cut into discs and bake.
If baking in the Aga, use the baking oven and bake for 10-12 minutes.