Preheat the oven to 350° F. Grease two cookie sheets and set aside.

In a large bowl, mix together the flour, cinnamon, baking soda, and salt. Set aside.

Using an electric mixer, cream together the butter, brown sugar, and granulated sugar for 1-2 minutes until well combined. Add the egg and vanilla extract and mix until smooth and creamy.

Slowly pour in the flour mixture and continue mixing until just combined, then mix in the oats, Reese's Pieces and chocolate chips, until fully combined, making sure to scrape down the sides of the bowl as needed.

Cover the cookie dough tightly with plastic wrap and refrigerate for 30 minutes.

Once the dough is chilled, remove it from the refrigerator. Scoop the cookie dough, form into 2" balls, and drop onto the prepared baking sheet, leaving space between each piece of cookie dough. Press down with your hand to flatten each ball of cookie dough slightly (make sure not to flatten them completely, just slightly).

Bake at 350° F for 10-12 minutes or until the edges of the cookies are lightly golden brown and the top is set. Remove from the oven and top with additional Reese's Pieces and chocolate chips, if desired. Let the cookies cool for about 10 minutes on the baking pan before eating. Enjoy!