Gather all your ingredients and tools on a clean countertop. Having everything ready will streamline the process.
In a medium saucepan, combine the 2 cups sugar, ½ cup milk, and 1 stick of butter. Heat over medium heat, stirring occasionally until the mixture comes to a rolling boil.
Once boiling, add in the ½ cup cocoa powder. Stir vigorously until everything is well blended and smooth.
Remove the saucepan from heat. Stir in 1 cup of smooth peanut butter and 1 tablespoon of vanilla extract until completely incorporated.
Next, fold in the 3 cups of quick oats. Make sure all the oats are coated with the chocolate mixture.
Using a spoon, drop tablespoon-sized amounts of the cookie mixture onto a baking sheet lined with parchment paper. Leave some space between each cookie, as they will spread slightly.
Allow the cookies to cool at room temperature for about 30 minutes or until they are set. If you're in a hurry, you can pop them in the refrigerator to speed up the setting process.