Prepare the Pan: Line a baking sheet or large plate with parchment or wax paper to prevent sticking.
Cook the Chocolate Mixture: In a saucepan over medium heat, stir together sugar, cocoa powder, butter, and milk. Bring to a full boil and let it cook for exactly 1 minute, stirring occasionally.
Add Peanut Butter and Vanilla: Remove from heat and quickly stir in peanut butter until melted. Add vanilla and mix until smooth and glossy.
Fold in the Oats: Stir in the oats until evenly coated with the chocolate mixture. Work quickly before it cools.
Shape the Cookies: Drop spoonfuls of the mixture onto the prepared pan, spacing them apart for even cooling.
Let Them Set: Let the cookies cool at room temperature for 30 minutes, or refrigerate for 10–15 minutes if you’re in a hurry.