Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly and achieve that perfect golden brown color.
In a medium bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon. Set aside.
In a large mixing bowl, cream the softened unsalted butter, light brown sugar, and granulated sugar together using an electric mixer until the mixture is light and fluffy. This should take about 2-3 minutes.
Add the large egg and vanilla extract to the butter-sugar mixture. Mix until well combined.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Be careful not to overmix; it’s okay if a few flour streaks remain.
Fold in the old-fashioned rolled oats, chopped walnuts, and semi-sweet chocolate chips using a spatula or wooden spoon. The dough will be thick and hearty.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a baking sheet lined with parchment paper, leaving space between each cookie. Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. Enjoy your homemade Oatmeal-Walnut-Chocolate Chip Cookies!
