In a medium saucepan, melt the butter and chocolate disks over medium heat, stirring constantly. Pour into a mixing bowl to cool for around 8-10 minutes.
Preheat the oven to 350˚F. Line a 9 inch square baking pan with parchment paper and set aside.
Whisk the sugar into the chocolate mixture. Add in the eggs one at a time, whisking after each addition. Whisk in the vanilla and cold brew concentrate.
Fold in the flow, cocoa powder and salt until fully incorporated.
Pour evenly into the prepared baking pan and bake in the preheated oven for 30-35 minutes, or until a toothpick inserted comes out clean.
Allow to cool in the pan on a wire rack. Cut into squares and store in an airtight container for up to 1 week.
Notes
You can freeze these brownies in the freezer for up to 2 months.