Cream the butter: Start by mixing the softened unsalted butter in a bowl for about 30 seconds until it's creamy and smooth.

Add the sugars: Combine both the light brown sugar and granulated sugar into the creamed butter. Beat them together until light and fluffy.

Incorporate the eggs: Crack in the large eggs one at a time. Mix well after each addition, then stir in the vanilla extract.

Whisk the dry ingredients: In a separate bowl, combine the all-purpose flour, cornstarch, baking soda, salt, and ground cinnamon. Whisk together until evenly mixed.

Combine the mixtures: Gradually add the dry mixture into the wet mixture. Stir just until combined to avoid overmixing.

Fold in the oats and extras: Gently fold in the old-fashioned rolled oats, ensuring they're evenly distributed. Add any optional ingredients.

Chill the dough: Cover the bowl with cookie dough and chill in the refrigerator for at least 30 minutes.

Preheat the oven: While the dough is chilling, preheat your oven to 375°F (190°C) and line baking sheets with parchment paper.

Drop the dough: Use a tablespoon to scoop out the chilled dough in 2-tablespoon portions onto the prepared baking sheets, spaced 2 inches apart.

Bake the cookies: Slide the baking sheets into the oven and bake for 10-12 minutes until the edges are golden brown and centers are soft.

Cool completely: Once baked, let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack.