Line 2 sheet pans with parchment paper.
In a medium saucepan, combine the granulated sugar, milk, salted butter, and cocoa powder over medium-high heat.
Bring the mixture to a full rolling boil. Once boiling, continue cooking for 1 minute.
Remove the saucepan from the heat.
Immediately stir in the quick-cooking oats, peanut butter, and vanilla extract until the oats are fully coated.
Scoop heaping tablespoons of the mixture onto the prepared sheet pans. Gently press down to shape them into cookies.
Allow the cookies to cool completely and set before serving.