1. Prepare the Cinnamon Sugar Filling:
- Mix the ingredients: In a small bowl, combine softened butter, brown sugar, flour, cinnamon, and a pinch of salt. Use a small rubber spatula or fork to mix until everything is well combined.
- Freeze the mixture: Place the cinnamon sugar mixture in the freezer for about 10 minutes. This helps firm up the butter and makes it easier to scoop and layer into the cookie dough later.
2. Make the Cookie Dough:
- Cream the butter and sugar: In a stand mixer or using a hand mixer, beat the butter and granulated sugar together until the mixture is light and creamy.
- Add the wet ingredients: Mix in the vanilla extract, egg, and egg yolk. Continue beating until well combined.
- Combine the dry ingredients: In a separate bowl, whisk together the flour, baking powder, cornstarch, and salt.
- Add the dry ingredients to the wet: Gradually add the dry ingredients to the butter-sugar mixture, mixing until fully combined.
3. Layer the Cinnamon Sugar Filling Into the Dough:
- Layer the dough: Once the dough is ready, remove half of it from the mixing bowl and set it aside. Spread the remaining dough out across the bottom of the mixing bowl.
- Add the cinnamon filling: Remove the cinnamon sugar mixture from the freezer. Use two spoons to scoop small pieces of the cinnamon mixture (about a teaspoon each) and place them evenly over the dough.
- Top with more dough: Take the dough you set aside earlier and press it into an even layer on top of the cinnamon filling.
- Add the remaining cinnamon filling: Spoon the rest of the cinnamon-sugar mixture on top of the second layer of dough.
4. Mix the Dough Together:
- Fold gently: Use a rubber spatula to gently fold the dough together. You want to stir just enough to create a swirled effect without fully incorporating the cinnamon filling. Leave some chunks of the cinnamon mixture intact to create a marbled effect.
5. Chill the Dough:
- Freeze the dough: Scoop the dough into two-tablespoon portions using a cookie scoop. Place the dough balls on a plate and freeze them for 10 minutes. This quick chill firms up the butter, preventing the cookies from spreading too much during baking.
6. Bake the Cookies:
- Preheat and prep: Preheat your oven to 350°F. Line two baking sheets with parchment paper or silicone baking mats.
- Place dough on baking sheets: Arrange the chilled dough balls on the prepared baking sheets, spacing them a couple of inches apart. For thicker cookies, leave the dough in higher mounds; for thinner cookies, flatten them slightly with your fingers.
- Bake: Bake the cookies for 10-12 minutes until they have spread slightly, the edges are golden brown, and the centers are just set. Some cookies might be a bit misshapen, which is fine because of the cinnamon filling melting out, but don’t worry—we’ve got you covered on shaping them.
7. Shape the Cookies (Optional):
- Option 1 – Glass or Cookie Cutter Method: As soon as the cookies come out of the oven, place a round glass, bowl, or cookie cutter (larger than the cookie) upside down over the top of the cookie. Gently swirl the glass in a circular motion, and the edges of the cookie will form into a perfect round shape.
- Option 2 – Spoon Method: Alternatively, use the back of a spoon to gently push the edges inward to create clean, round shapes.
8. Cool and Enjoy:
- Cool on wire racks: Let the cookies cool on the baking sheets for a few minutes before transferring them to wire racks. The cookies will firm up as they cool, ensuring a chewy, soft texture in the center and a slightly crisp edge.