Line two cookie sheets with parchment paper. Set aside. Bring butter, sugar, cocoa, milk and salt to a boil on medium heat. Boil for 2 minutes. Stir in peanut butter, oats and vanilla. Stir until completely combined.
Using a small cookie scoop (1 ½ tablespoons), drop onto prepared pans. Let them set for one hour. Store in an airtight container. They are best kept in the fridge and keep for a few days.
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Notes
The original recipe calls for twice as much sugar, but I found them too sweet that way and cut it in half. Feel free to adjust to your taste - use up to 2 cups of sugar if you prefer an extra sweet cookie.