Best Practice at cutting butter and sugar in Peanut Butter balls
I like peanut butter, but the thing with using neat peanut butter in recipes is that it can be very cloying and weld your mouth shut. Mixing peanut butter with butter helps loosen it up a bit, the sugar is there to add flavor back and keep it sweet as butter is a savory, and not a sweet flavor. Peanut butter will roll in balls just fine without any extras, they are just there to improve the eating experience. 

Having a big ball of peanut butter in the center of a cupcake is problematic because cupcake batter is fairly slack, however brownie batter is thicker so I it's less likely to run out the bottom. 

Freezing the peanut butter is a good thought, however it is likely to keep the center from cooking - the outside of the brownie will be done but the center will be very underdone which is probably not what you are looking for. I'd suggest trying the balls at refrigerator temperature first.