Preheat the oven to 350°F. Spray an 9x13-inch baking pan with cooking spray or line with parchment paper
On medium speed in a stand mixer or with a hand mixer, cream the butter, peanut butter and sugars for 2 minutes until fluffy
Add in the eggs and vanilla extract and mix on medium for 1 more minute
Add in the flour and salt and mix on low until just combined (~20 seconds)
Fold in the chocolate chips
Pour the batter into the prepared pan and smooth with a spatula to form an even layer
Scatter the M&Ms on top and eye sprinkles, if using
Bake for 25-30 minutes until the center is no longer jiggly but soft (note this bake time is for a 9x13 inch metal pan - if using a glass pan or smaller pan, the time will need to be increased)
After baking, add three Halloween Reese's to the top if using. Let them sit for ~1 minute to soften and then press in two eyes on top of each peanut butter cup. Let cool for ~1 hour before cutting