Line two baking sheets with parchment paper or wax paper.
In a medium saucepan, whisk together the sugar, milk, butter, and cocoa powder and place over medium heat. Stir occasionally until the mixture reaches a boil.
Once it’s boiling, stop stirring and allow the mixture to boil for exactly 60 seconds (set a timer!), then remove from heat.
Add the oats, peanut butter, and vanilla. Stir together with a silicone spatula until well combined.
Use a cookie scoop or spoon to drop heaps of the mixture onto the lined baking sheets (about 2 tablespoons each).
Let the cookies cool until set, 20 to 30 minutes.
