Line an 8×8 or 9×9 pan with aluminum foil and spray it with cooking spray and set it aside. In a medium saucepan, combine butter, sugars, cocoa powder, milk and heat over medium-high to high heat, stirring to encourage the ingredients to melt and combine. Once all items have melted and combined, allow the mixture to boil for 90 seconds, stirring continuously.
After 90 seconds, turn the heat off and add the peanut butter and vanilla extract (careful when you add the vanilla, it could bubble up a bit on you) and stir until smooth. Then add the oats and stir to coat.
Pour batter into prepared pan, spreading it evenly into the pan. If adding any optional chips to the top of the bars, add them now; pressing them into the mixture lightly with a spatula. (If you press them into the batter too deeply, they will probably melt; not bad, but not what I intended for which is why I kept them at the surface level to avoid complete melting)
Place pan into the freezer and freeze at least 30 minutes, or until bars have set up, before slicing and serving. (I take the bars out of the pan by holding the edge of the foil and place the whole foil “slab” onto a cutting board rather than cutting them inside the pan because I can be neater on a cutting board and I also don’t carve up the bottom of my pan with knife marks).