Preheat oven to 350°F with racks near the center. Line two baking sheets with parchment paper and set aside.
Cinnamon sugar filling: Mix together all ingredients until well combined. Place in the freezer for about 10 minutes while you prepare the cookie dough.
Cookies: Using a hand mixer, beat butter and sugar until light and creamy. Add vanilla, egg, and egg yolk, and beat until combined. In a separate bowl, combine flour, baking powder, cornstarch, and salt. Slowly add the dry ingredients to the wet ingredients and beat until combined.
Remove half of the dough from the mixing bowl and spread the remaining dough out across the bottom of the mixing bowl. Remove the cinnamon mixture from the freezer and scoop small pieces of the cinnamon mixture and place them all over the dough in the mixing bowl. Once you have used half of the cinnamon mixture, get the cookie dough that you have set aside, and spread it on top of the cinnamon and dough in the mixing bowl. Proceed to add the remaining cinnamon mixture all over the top of that piece of dough.
Once you have your dough layered, fold and mix it just barely. Scoop the dough into two-tablespoon portions using a cookie scoop. Place the dough balls on a plate and freeze for 10 minutes. Place dough balls a couple inches apart on parchment lined baking sheets. Bake for 10-12 minutes, until the cookies have spread, the edges are beginning to brown, and the center is still slightly underdone. Place baking sheets on wire racks and allow cookies to cool completely.