Prepare the Baking Sheet: Line a baking sheet with parchment paper or wax paper. Have all of your ingredients measured and ready to go.

Combine First 4 Ingredients: In a medium saucepan, combine the butter, milk, white sugar, and brown sugar. Bring the mixture to a boil over medium heat, stirring frequently to dissolve the sugars and melt the butter.

Boil the Mixture: Once the mixture comes to a rolling boil, allow it to boil gently for about 1 to 2 minutes without stirring. This step is crucial as it ensures the cookies will set properly.

Remove from Heat: After boiling, remove the saucepan from the heat.

Add Remaining Ingredients: Stir in the cocoa powder, peanut butter, vanilla extract, and cinnamon (if using) until the peanut butter is fully melted and the mixture is smooth.

Add Oats: Quickly add the oats to the mixture and stir until they are fully coated.

Form the Cookies: Drop spoonfuls of the mixture onto the prepared baking sheet. You can use a cookie scoop or two spoons to shape the cookies.

Let Set: Allow the cookies to set at room temperature for about 20 to 30 minutes or until firm. You can also place them in the refrigerator to speed up the setting process.