Preheat your oven to 350°F with the racks near the center. Line two baking sheets with parchment paper and set them aside, so they're ready for when the dough is chilled and shaped.

Combine 4 tablespoons unsalted butter (softened), 1/3 cup packed brown sugar, 1 1/2 tablespoons all-purpose flour, 2 1/2 teaspoons ground cinnamon, and a pinch of salt in a small bowl. Mix with a small rubber spatula or fork until well combined into a thick paste. Place the cinnamon mixture in the freezer for about 10 minutes to firm up while you prepare the cookie dough.

In a large bowl, beat 1 cup unsalted butter (softened but cool) and 1 1/3 cups granulated sugar with a hand mixer or stand mixer fitted with the paddle attachment until light and creamy. Add 1 3/4 teaspoons vanilla extract, 1 large egg, and 1 large egg yolk, beating until combined and scraping down the sides as needed. In a separate bowl, whisk together 2 3/4 cups all-purpose flour, 3/4 teaspoon baking powder, 1 teaspoon cornstarch, and 1/4 teaspoon salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined and a dough forms.

Remove half of the cookie dough from the bowl and set aside. Spread or press the remaining dough out across the bottom of the mixing bowl. Take the cinnamon sugar mixture from the freezer and use two spoons to scoop small pieces (about a teaspoon each), scattering half of it over the dough in the bowl. Place the reserved half of cookie dough over the first layer, spreading it into an even layer with your hands. Scatter the remaining cinnamon sugar mixture in small pieces over the top. Now, fold and marble the dough by making two folds with a spatula, scraping up and over, then give one or two gentle stirs by hand. Avoid overmixing—leave some cinnamon-touched streaks and pockets intact for the best swirled effect. I love leaving visible chunks of cinnamon sugar; it creates the best gooey pockets in the final cookies.

Using a cookie scoop, portion the marbled dough into two-tablespoon balls, ensuring each scoop contains both dough and cinnamon mixture. Place the dough balls onto a plate and freeze them for 10 minutes, which helps firm up the butter and prevents excess spreading during baking. If you're working in batches, keep any extra dough balls in the refrigerator until it's their turn to bake.

Arrange the chilled dough balls a couple inches apart on the prepared baking sheets. For thicker cookies, leave the dough balls in higher mounds; for thinner cookies, gently pat them down. Bake in the preheated oven for 10-12 minutes, or until the cookies have spread, the edges are just beginning to brown, and the centers are still slightly underdone. Some cookies may look a little uneven because of the cinnamon pockets, but that’s part of the charm. I suggest gently nudging misshapen cookies into a perfect circle with a spoon as soon as they come out of the oven for bakery-style results.

Let the baked cookies cool completely on the baking sheets placed over wire racks. As they cool, the cookies will finish setting up and become perfectly chewy with cinnamon swirls throughout.