Gather all your ingredients and set up your workspace with parchment paper ready on a tray.
Combine the butter, sugar, cocoa powder, and milk in a medium saucepan. Turn the heat to medium and stir constantly as the mixture begins to melt and bubble, filling your kitchen with a rich chocolate aroma.
Let the mixture boil gently for exactly one minute, maintaining a steady simmer while stirring. You'll notice it thickening slightly and becoming glossy, with a crackling sound as it bubbles.
Remove the saucepan from heat and stir in the vanilla extract. Then, add the oats all at once, folding them into the warm chocolate mixture until they are evenly coated and the mixture becomes thick and sticky.
Using a spoon or small scoop, drop spoonfuls of the mixture onto your prepared parchment paper. Flatten each cookie slightly with the back of your spoon for a more uniform shape.
Allow the cookies to sit at room temperature for about 30 minutes to an hour until they firm up, or place them in the fridge for 15-20 minutes to speed up the process.
Once set and firm, peel the cookies gently from the parchment paper. They will have a shiny, chocolate coating and a chewy, oat-filled interior. Enjoy immediately or store in an airtight container at room temperature for up to a week.
