Preheat the oven to 350F/180 C. Prepare an 8x8 square baking pan with parchment paper.
In a microwave-safe bowl, melt the butter, chocolate chips, and miso paste together, stirring every 20-30 seconds, until smooth. Set aside.
In a large bowl, whisk together the sugar, eggs, and vanilla until light and fluffy, about 1-2 minutes.
Stream the melted chocolate mixture into the mixing bowl and whisk until combined. Then, fold in the flour, black cocoa powder, and salt. When it's almost combined, fold in the milk chocolate.
Pour the brownie batter into the prepared baking pan. Bake for 30-40 minutes, or until the middle of the brownies is puffed and raised, and a toothpick inserted into the center comes out covered in a few moist crumbs.
Let the brownies cool completely, then use a sharp knife, wiping it clean in between cuts, to cut it into 16 small brownies or 9 large brownies. Enjoy!