A spherical tray should work. Make sure it has enough mass to make its gravitational field hold on to the batter and suspend it in the oven using magnets.
Panko bread dough is placed between two metal surfaces, and is cooked by running an electric current through it. This avoids any crust forming, causing it to be very uniform.
I suppose you could bake a brownie sphere in microgravity. That might be hard to get the center cooked without burning the surface though. Maybe a metal ball as a pan that you could coat with batter.