Line a 11 x 7 inches pan with parchment paper and set aside.
In a sauce pan over medium heat, combine butter, brown sugar, sugar milk and salt. Whisking constantly bring the mixture to a boil. The mixture will boil quickly so keep an eye on it. Boil for 1 minute then remove the pan from a heat.
Add peanut butter and vanilla and whisk until combine.
Using a spoon, fold in the oats. Do not add all oats at once. Add 2 cups, stir well, then add ¼ cup at the time if needed. I use 2 1/4 cups. even though the mixture looks looks wet it will harden up.
Pour mixture into pan and spread it out evenly. Place in fridge while you make the frosting.
To make the chocolate frosting, combine ¼ cup peanut butter and chocolate chips in a microwave safe bowl and microwave for 30 seconds then stir. Microwave again for 15 seconds intervals. I only need 45 seconds and it is perfect.
Spread chocolate frosting over the peanut butter oatmeal bars, sprinkle with chopped Reese’s cups and refrigerate for 1 -2 hours before cutting.