There are two options if you have an 8x8/9x9 brownie mix or recipe. First, you can use two boxes or double the recipe, then make the bars as called for. The other option, which is my preference, is to make the 8x8/9x9 mix or recipe as it's called for, then reduce the ingredients for the peanut butter filling and ganache by about 1/3. You can cut the ingredients in half, but you will have thin layers.