How do I increase the amount of store bought peanut butter from 1/2 cup to 1 cup in a peanut butter brownie recipe?
It is possible to substitute regular butter with any nut butter (like peanut butter), it will add a nutty flavour of course but this shouldn't be an issue since there's already peanut butter included. The difference is the fat content however, have a look at this post that explains it quite well.
They mentioning adding oil to the peanut butter and mixing it to get a more butter-like effect. So you could add 1/2 cup of peanut butter (this is in addition to the 1/2 cup that's already included making it 1 cup of peanut butter total) with 1/2 cup of oil (I recommend coconut for best flavour with baking) and change the amount of butter to 1/4 cup (removing 1 cup as the substitute).
1 1/4 cup butter = 1/2 cup peanut butter + 1/2 cup oil + 1/4 cup of butter
This gets pretty complicated but is likely the most accurate. If you're open to experimenting a bit, I would honestly just try it without the oil and see how they come out, hopefully the difference in texture isn't noticeable enough.