In a medium saucepan, melt the butter over medium heat.
Stir in sugar, milk, and cocoa. Bring to a rolling boil and cook for 1 minute, stirring constantly.
Remove from heat and quickly stir in the peanut butter, oats, and vanilla until everything is well combined.
Spoon mixture into muffin liners or silicone molds, pressing down gently.
Chill in the fridge for at least 30 minutes before serving. Store leftovers in the refrigerator.