Preheat the oven to 350 F and prepare a cooking sheet with nonstick cooking spray or parchment paper.
Combine butter, sugar, brown sugar, and egg into a large mixing bowl. Beat on low with an electric hand mixer or stand mixer until smooth and creamy.
Add peanut butter and vanilla. Mix on low until combined. Set aside.
In a separate large bowl mix together all purpose flour, quick oats, baking soda, and salt.
Whisk the dry Ingredients until combine. Next slowly add in to wet ingredients. Add ½ or 1 cup at a time, beat on low until combined, and then continue with the next batch. (Mix until ingredients are almost combined, a few lumps here and there are fine.)
Stir in M&Ms. Place your dough in the fridge to chill for 2 hours in the fridge. It is important to chill the batter between the batches too.
The batter will resemble playdough and should not be sticky after chilling. Scoop 2 tablespoonfuls of batter, rolling it into about 2 inch balls.
Arrange the cookie balls (2 tablespoonful sized balls) on a baking sheet lined with parchment paper at least 2 inches apart.
Bake for 10-12 minutes (the time varies due to different ovens).
The cookies will have a very soft center inside. Once they cool down they will set well and the texture inside remains soft, chewy and buttery.
Let the cookies stand in the pan for 5-10 minutes before transferring to a wire rack.