Line two baking sheets with parchment paper.

Melt the butter in a medium saucepan over medium heat. Add the sugar, milk, and cocoa powder and whisk until smooth. Bring to a rolling boil, adjusting the heat as needed to maintain a full boil for 2 minutes.

Remove from the heat. Add the peanut butter and vanilla and stir until smooth, then add the oats and mix until well combined. Use a 2-tablespoon cookie scoop to scoop the oat mixture onto the prepared baking sheets, flattening the cookies slightly if desired.

Chill in the refrigerator for at least 30 minutes to firm up. The cookies will become firmer the longer they chill.

Store in an airtight container in the refrigerator for up to a week.