Prepare all ingredients in advance and line two baking sheets with parchment paper.
In a large saucepan over medium heat, combine butter, sugar, milk, and cocoa powder. Stir frequently until melted and smooth.
Bring the mixture to a rolling boil and let it boil for exactly 60 seconds while stirring occasionally.
Remove from heat and stir in peanut butter and vanilla extract until fully combined.
Add oats and mix until everything is well coated.
Drop spoonfuls of the Classic No Bake Cookies mixture onto prepared baking sheets.
Let cool for 20–30 minutes until set, then serve and enjoy.