In a medium saucepan over medium heat, combine the butter, cocoa, sugar, milk, and salt together. Bring mixture to a boil and boil for 1 minute, stirring occasionally.
Remove mixture from heat and stir in oatmeal, peanut butter, and vanilla until combined. The mixture will begin to thicken.
Using a small ice cream scoop, drop 1-2 tbsp of mixture onto a parchment or wax paper-lined baking sheet. Flatten cookies slightly.
Sprinkle each cookie with a touch of sea salt and allow to cool and harden completely (about 1 hour)