Line two baking sheets with parchment. Measure everything before touching the stove, this moves fast and waits for no one.

Combine butter, milk, sugar, cocoa, salt, and cream of tartar in a medium heavy-bottomed saucepan over medium-low heat. Whisk until the butter melts and the sugar fully dissolves. Don't rush this part, it matters.

Raise the heat to medium. Bring to a full rolling boil, then hold it for exactly 1 minute (or watch for 230°F–234°F).

Pull from the heat immediately. Stir in peanut butter and vanilla until smooth and glossy.

Fold in oats. Now let it sit for 5 minutes, this gives the oats time to absorb moisture and thicken before you scoop. Skip this, and they'll spread flat.

Drop heaping tablespoons onto parchment fast, it sets as it cools.

Leave them alone for 30 minutes at room temperature. Walk away. Don't poke them.