In a medium/large bowl start by mixing together the dry ingredients including the flour, cinnamon, salt, baking soda and old-fashioned rolled oats. Set this bowl aside.
Beat the butter, light brown sugar and sugar together. You can use a large bowl with a hand mixer or use a stand mixer to beat these ingredients together.
Beat the egg and vanilla extract in the butter mixture.
Add the dry ingredients to the butter mixture a little at a time until combined. Don't overmix!
Add the chocolate chips or chocolate chunks to the dough and stir together.
Cover the bowl and refrigerate the dough for 20 minutes.
Preheat the oven to 350 degrees and line a cookie sheet with parchment paper or silicone baking mat.
Use a muffin scoop to scoop the dough on the cookie sheet. Bake for 9-12 minutes or until the cookies are slightly brown around the edges.
Allow the cookies to cool for a few minutes before transfering them to a wire rack.