Add the butter, sugars, cocoa powder, and milk to a medium sauce pan. Measure out all of the other ingredients and have them ready to go near the stove.
Cook the butter mixture over medium-high heat, stirring, until the butter melts and the mixture comes to a full rolling boil even in the center (not just around the edges). Once the mixture is at a rolling boil, stop stirring and cook for exactly 1 minute.
Remove from heat and working quickly, stir in the peanut butter, followed by the oats, vanilla, and salt, until completely mixed.
Let the mixture rest for 2-3 minutes for the oats to soak up a bit of moisture while you line a baking sheet with parchment paper.
Scoop 2-tablespoon-sized balls of the mixture onto the prepared baking sheet. If your cookie spreads too much, let the mixture cool for another minute. Working quickly, scoop all of the cookies onto the baking sheet with a cookie scoop or spoon, shaping them into mounds, if needed.
Before removing the cookies from the baking sheet, let them cool completely until firm to the touch, 1-2 hours at room temperature or 15-30 minutes in the fridge.
Store on the counter in an air-tight container for up to 3 days or in the fridge for a week. Enjoy straight from the fridge or at room temperature.
