Preheat the oven to 180 C/160 C fan. Grease and line a 20cm square baking tin.
Melt the butter in a small saucepan until it begins to foam. Continue cooking, stirring regularly, until golden brown and starting to smell nutty - this is the milk solids toasting. Set aside to cool.
Once cool, whisk in the sugar, miso, eggs and vanilla.
Whisk together the flour and baking powder in a medium bowl, then fold into the wet batter, followed by your chopped chocolate and walnuts.
Transfer to the baking tin and bake for 30 - 35 minutes until the top is crisp and golden and just beginning to crack. Allow to cool completely then slice into squares.
Will keep in an airtight container for 3 - 4 days.