Warm your oven to 350°F, positioning the racks towards the center. Prepare two baking sheets by lining them with parchment paper.
For the Cinnamon Sugar Filling:
Combine all filling ingredients using a small rubber spatula or fork until they’re thoroughly mixed. Freeze this mixture for about 10 minutes as you start on the cookie dough.
Making the Cookies:
- With a hand mixer or a stand mixer equipped with a paddle attachment, cream the butter and sugar together until the mixture is light and fluffy. Incorporate the vanilla, egg, and egg yolk, continuing to mix until everything is well combined. Remember to scrape down the sides of the bowl as needed.
- In a different bowl, whisk together the flour, baking powder, cornstarch, and salt. Gradually blend these dry ingredients into the wet mixture until just combined.
- Take out half of the cookie dough and flatten the remaining dough across the bottom of your mixing bowl.
- Retrieve the cinnamon mixture from the freezer. Using two spoons, create small teaspoon-sized dollops of this mixture and scatter them over the dough in the bowl.
- Use the reserved half of the cookie dough to form another layer over the cinnamon bits, pressing down to even it out. Sprinkle the rest of the cinnamon mixture on top, creating a second layer of flavor.
- Gently fold and mix the dough with a rubber spatula to barely combine, ensuring some cinnamon mixture remains in chunks while some are swirled into the dough. Avoid overmixing to maintain the marbled effect.
- Using a cookie scoop, form the dough into two-tablespoon portions, making sure each has a good mix of dough and cinnamon.
- Chill the dough balls on a plate in the freezer for 10 minutes to firm up the butter, preventing excessive spreading during baking.
- Space the dough balls a few inches apart on the prepared baking sheets. For thicker cookies, leave the balls as is; for thinner cookies, slightly flatten them. If you have extra dough balls, keep them in the refrigerator until ready to bake.
- Bake in the preheated oven for 10-12 minutes. The cookies should spread slightly, with edges turning golden but centers remaining soft.
- If any cookies come out misshapen, immediately reshape them using the back of a spoon or a round object to press the edges inward, creating a perfect round shape.
- Let the cookies cool on the baking sheets set over wire racks until they firm up.