Using a food mixer, beat together 150g of unsalted butter, 100g of caster sugar and 175g of soft brown sugar. Break 1 large egg into a small mixing bowl and lightly beat with a fork.
When the butter and sugars are light and creamy, mix in ½ tsp of sea salt and the beaten egg. Stir in 90g of white miso paste. Roughly chop and add 150g of walnut halves.
Mix 300g of plain flour and 1½ tsp of baking powder and add to the dough, stirring briefly and taking care not to over-mix. Stop as soon as the flour is incorporated. Transfer the dough to a bowl, cover and refrigerate for 1 hour.
Heat the oven to 190C/gas mark 5. Put 50g of soft brown sugar into a small dish. Break off pieces of the chilled dough (aim for each to weigh 45g) and roll into balls.
Roll the balls in the soft brown sugar, making sure they are covered all over, then place on a parchment-lined baking sheet a good 4cm apart. Press a walnut half on to each, then bake in the preheated oven for 10 minutes until pale gold. Remove from the oven, leave to settle for 10 minutes, then lift on to cooling rack using a wide spatula. They are nuttily fragrant warm from the oven, but will keep in a tin for a few days.
Makes about 18 cookies. Ready in 30 minutes, plus resting time.
• After 10 minutes of baking, the cookies may appear quite soft, but they will crisp up on cooling.
• You will get a great cookie if you rest the dough for an hour or two first, but an overnight rest in the fridge will give you a more open-textured cookie with a crackling crust.
• When you take the baked cookies from the oven, bang the tray on the counter or table. It will help the surface of the cookies crackle, like a ginger nut or Cornish fairing.