Add the sugar, butter, and cocoa to a large pot. Bring to a gentle boil on low heat. Stir until the sugar is melted and the ingredients are incorporated.
Let the chocolate mixture cool for 1-2 minutes then mix in the peanut butter, milk, and vanilla.
Pour the oatmeal into the mixture and stir until evenly coated.
Using a cookie scoop or spoon, drop the cookie mixture onto a baking sheet lined with waxed paper and let fully cool.
Once cool, remove the cookies to an airtight container.
Notes
Storing: Once the cookies have cooled, they can be stored in an air-tight container for 4-5 days, or frozen for up to 3 months.