Prepare Baking Sheet: Line a baking sheet with parchment paper or wax paper. Set aside.
Melt Ingredients: In a medium saucepan, combine butter, sugar, cocoa powder, and milk. Heat over medium heat, stirring constantly until the mixture boils. Let it boil for 1 minute, then remove from heat.
Add Peanut Butter & Vanilla: Stir in peanut butter and vanilla extract until smooth.
Mix in Oats: Fold in the oats until fully coated.
Shape Cookies: Drop tablespoonfuls of the mixture onto the prepared baking sheet. Flatten slightly with the back of a spoon.
Chill: Let the cookies cool at room temperature for 30 minutes (or refrigerate for 15 mins) until firm.