In a saucepan, melt the butter over medium heat. Stir in the sugar, milk, and cocoa powder. Bring to a rapid boil and cook for 1 minute while stirring constantly.
Remove from heat and stir in the peanut butter and vanilla extract until smooth.
Add the oats, stirring to coat them completely.
Drop by spoonfuls onto wax paper and let cool until set.
Notes
Store cookies in an airtight container for up to a week.