Preheat the oven to 350°F. Lightly spray a mini muffin pan with cooking spray and set aside.
In a large bowl, mix together the eggs, cocoa powder, baking powder, cold brew concentrate and vanilla until fully combined. Mix in the sugar and butter. Fold in the flour until just combined.
Scoop the brownie batter into the prepared mini muffin pan and bake in the preheated oven for around 12 minutes, or until a toothpick inserted comes out clean.
Allow the brownies to cool for around 10 minutes in the pan, then transfer to wire racks to cool completely.