Line a 9x9-inch dish with aluminum foil; enough to hang over side to help with easy removal. Lightly coat with cooking spray and set aside. Measure out all ingredients and have ready because this moves fast.
In a medium saucepan add butter, milk and sugar; stir to melt butter. Increase to medium high and constantly stirring, bring to a boil. Cook for 2 minutes. Remove from heat and stir in peanut butter and vanilla until well mixed. Add oats and stir until everything is well mixed. Pour into prepared dish and smooth out on top. Cool for about 30 minutes until easy to handle. Lift out with foil and with a knife cut into 24 pieces. Store leftovers in a zip lock bag.