Add the butter, milk, cocoa powder, and sugar to a saucepan over medium heat.
Whisk to combine and heat the mixture until the sugar has dissolved.
Remove the saucepan from the heat, and stir in the vanilla, peanut butter and oats until well combined.
Drop spoonfuls of the mixture on the parchment-lined baking sheet, and chill until the cookies have hardened.
Notes
Measure the ingredients accurately. When measuring ingredients like cocoa powder and oatmeal, use a spoon to gently scoop into the cup, leveling it off with a knife or finger.
Adjust the butter. If your cookies are too soft, increase the butter or add more peanut butter.
Use a cookie scoop. For perfectly round cookies, portion out the mixture onto parchment paper-lined baking sheets with a cookie scoop.
Press down on each cookie mound with your fingertips to flatten them slightly.