Line an 8×8 inch square baking pan with parchment paper.
Combine butter, sugar, cocoa powder, and milk in a medium-sized saucepan.
Heat over medium-high heat while whisking until it reaches a gentle rolling boil.
Boil the mixture for exactly one minute then remove from heat.
Stir in the peanut butter and vanilla extract until the mixture is smooth.
Fold in the quick oats until the ingredients are fully incorporated.
Spread the mixture into the prepared pan and press down evenly.
Chill in the refrigerator for at least 60 minutes until set before slicing into bars.