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.
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Notes
Milk options: You can use a variety of different types! 2% milk, whole milk, evaporated milk, or a nondairy option like almond milk or oat milk will work. Just be sure it isn't fat-free.
When boiling the sugar mixture, use a thermometer if needed: If you are cooking at a high altitude or prefer to be very precise, check the mixture with an instant-read thermometer. The cocoa and sugar mixture should reach 230-234°F before you remove it from the heat. At a higher altitude, this can take longer than 60 seconds.
Storage: The cookies will keep well in an airtight container for up to a week at room temperature or 2 weeks in the fridge.