In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. Whisk until combined.
Add the softened butter and sugar to a separate large mixing bowl. Using a hand mixer, mix until light and fluffy and fully combined.
Add the egg and vanilla extract to the butter and sugar mixture. Mix with the hand mixer until fully combined.
Add the dry ingredients to the wet ingredients. Mix until just combined.
Gently fold the M&M’s and sprinkles into the cookie dough to evenly distribute.
Add the cookie dough to the prepared baking pan (metal pan) and using a rubber spatula or your hands work the dough so it covers the entire surface of the pan.
Bake for 20-25 minutes until the edges are golden brown and a toothpick comes out with a few crumbs. Remove and allow to cool for 5-10 minutes before cutting.