Line two cookie sheets with parchment paper or a silicone mat.
In a medium-sized saucepan over medium heat combine the butter, sugar, cocoa, and milk. Cook, stirring frequently until it reaches a full boil. Allow mixture to boil for 1 minute without stirring.
Remove from heat, immediately add in vanilla, peanut butter, and oats. Stir the mixture until well combined then scoop (about 2 tablespoons size) onto lined cookie sheets.
Allow the cookies to rest at room temperature until set, about 20-30 minutes.
For Glaze:
In a small saucepan add all of the ingredients and stir until there is a smooth consistency. Add more milk if too thick and more powdered sugar if too thin.