Can I replace a food processor in this brownie recipe?
No, you can use the blender.  Use the slowest speed and manually do short pulses (1 second on, 2 seconds off).

You may have to use a spatula and mix it to get an even result. The pauses are so that the food doesn't get too hot (friction from the blades can actually boil things).  Stop early,  it's easy to make an unrecognizable paste in the blender.