Freezing Peanut Butter Balls
The cold temperature of the peanut butter will completely change the cooking process of the brownie, as it will be cooled from the inside while heated from the outside. I don't think this is a good thing. You could try room temperature peanut butter, but PB has a melting point around 150F (depending on the emulsifiers), and would probably melt before your brownie was fully cooked. 

Unfortunately I do not think this will work.