Line two large baking sheets with parchment paper or silicone baking mats. Set aside.
Place the butter, granulated sugar, milk, and cocoa powder in a medium saucepan. Over medium heat, bring to a boil, stirring occasionally. Once the mixture is at a rolling boil, boil for one minute.
Remove the pan from the heat and stir in the peanut butter, vanilla, and salt until the peanut butter is melted and smooth.
Stir in the oats.
Drop spoonfuls of the mixture (about 2 tablespoons per cookie) onto the prepared baking sheets.
Let sit at room temperature until cooled and hardened, about 20 to 30 minutes.