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 imagine you could do the same with brownies.

Sous vide maybe?

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.

Well a circle has the best ratio for your liking i think. But maybe ask your local magician for some un edged Brownies ;).