Step 1: Prepare the Cinnamon Sugar Filling

In a small bowl, mix the softened butter, brown sugar, flour, cinnamon, and salt until well combined. Place the mixture in the freezer for about 10 minutes while you prepare the cookie dough.

Step 2: Make the Cookie Dough

Using a stand mixer or hand mixer, beat the butter and sugar together until light and creamy. Add the vanilla, egg, and egg yolk, beating until fully combined. Scrape down the sides of the bowl as needed.

In a separate bowl, whisk together the flour, baking powder, cornstarch, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Step 3: Layer the Dough with Cinnamon Mixture

Divide the cookie dough in half. Spread one half across the bottom of the mixing bowl. Remove the cinnamon mixture from the freezer and break it into small teaspoon-sized pieces. Scatter half of these pieces over the dough.

Take the remaining cookie dough and press it gently over the cinnamon-filled layer. Repeat the process by adding the remaining cinnamon mixture on top.

Step 4: Swirl the Dough

Using a rubber spatula, fold the dough twice, scraping up and over, followed by one to two gentle stirs. The goal is to have visible cinnamon swirls, not fully incorporated into the dough.

Step 5: Scoop and Chill the Dough

Using a cookie scoop, portion out the dough into two-tablespoon-sized balls. Ensure each scoop has a good mix of dough and cinnamon. Place the dough balls on a plate and freeze for 10 minutes to prevent excessive spreading.

Step 6: Bake the Cookies

Preheat the oven to 350°F. Line two baking sheets with parchment paper. Place the chilled dough balls a couple of inches apart on the baking sheets.

For thicker cookies, leave them as mounds. For thinner cookies, gently press them down. Bake for 10-12 minutes, or until the edges start to brown and the centers remain slightly underdone.

Step 7: Cool and Enjoy

Let the cookies cool on the baking sheets for a few minutes before transferring them to wire racks. They will firm up as they cool.