You can make this in the microwave! Place the butter and peanut butter in a large microwavable bowl; cover with a paper towel and microwave on high for 2 minutes. Stir, microwave for 1 more minute, then proceed with the rest of the recipe as written.
If you don't have an 8-inch square pan, you can use a 9-inch square pan, but the fudge will not be as high.
The fudge can be stored in an airtight container or in a freezer ziploc bag for up to 3 months.