Preheat oven to 350 degrees F (176 C) and spray 7-8 standard-sized muffin tins with cooking spray (amount as original recipe is written // adjust if altering batch size). Alternatively, fit each with a paper liner.
Prepare flax eggs in a small bowl and let rest for 5 minutes.
Place butter in a large mixing bowl and melt in the microwave. Then stir in the flax egg, sugar, vanilla, baking powder, salt and cocoa powder. Whisk to combine. Lastly, add the flour, then fold in any mix-ins - chocolate chips or walnuts would be excellent here.
Scoop batter evenly into 7-8 muffin tins (as original recipe is written) until 3/4 full and bake on the middle rack for 22-26 minutes, or until the brownies start to pull away from the sides and they spring back slightly to the touch. Be careful not to overcook or they will get crumbly. Remove from oven and let rest in the tin for 5 minutes before removing to cool completely on a plate or cooling rack.
Store in an airtight container to keep fresh for a couple days. Freeze for long-term storage.