Line a 8x8" pan with parchment paper or foil. Set aside. Place the white chocolate, sweetened condensed milk and peanut butter in a microwave safe bowl. Microwave for 1 minute. Stir for 15 seconds. If there's still chunks microwave another 15-30 seconds. Stir until smooth. Add in vanilla if desired.
Pour the fudge into the pan and then smooth out with a spatula. Try to move quickly as it tends to thicken fast. Place in the refrigerator until firm.
Once the fudge has set, cut into 1-inch squares.
Notes
Storage: Stored in an airtight container on the counter for 5-7 days. This fudge will stay fresh 2-3 weeks in the refrigerator and 3+ months in the freezer.