Line 2 rimmed baking sheets with parchment paper.

Stir together oats, coconut, peanut butter, and vanilla in a large bowl with a spatula until well combined.

Combine sugar, milk, butter, and cocoa powder in a saucepan over medium heat. Stir occasionally and gradually bring mixture to a boil. Then, stir constantly and boil for no longer than 90 to 120 seconds; if it boils for too long the cookies will turn out crumbly. Remove from the heat and pour over the oat mixture; stir until well combined.

Drop large scoops batter onto the prepared baking sheets. Spray the back of a spoon with cooking spray, and use it to flatten the scoops; respray as needed. Let cool to room temperature.