Line an 8x8 baking pan with parchment paper, leaving an extra inch on each side of the pan, then set the pan aside until the base of the bars is ready.
In a large bowl, or using a stand mixer, beat the butter and the sugars together. Once they have combined, add in the peanut butter and continue mixing for about 30 seconds.
Add the powdered oats, vanilla, salt, and oats, mixing the dough together. Stir in the ⅔ cup of M and M’s, until they are evenly distributed throughout the dough.
Press the mixture in the lined pan, spreading it evenly. Press it firmly into the bottom of the pan to avoid any gaps.
In a small, microwave safe bowl, melt the chocolate chips and the heavy whipping cream, for about one minute. Stir until the chips are completely dissolved and it is a thick chocolate syrup. Microwave for an additional 30 seconds if the chips aren’t melting.
Pour the chocolate sauce over the oat mixture, spreading it evenly. Sprinkle the remaining ⅓ cup of M and M’s over the top of the chocolate.
Cover and place in the refrigerator for about 2-3 hours, allowing the chocolate to set. Cut and serve.