Preheat oven: Preheat to 350°F (177°C). Line two baking sheets with parchment paper.

Make filling: Combine butter, brown sugar, flour, cinnamon, and salt. Mix until smooth. Freeze 10 minutes while preparing dough.

Cream dough base: Beat butter and sugar together until light and fluffy. Add vanilla, egg, and egg yolk; mix until blended.

Mix dry ingredients: In a separate bowl, whisk flour, baking powder, cornstarch, and salt. Gradually add to wet mixture until combined.

Layer and swirl: Flatten half the dough in the mixing bowl. Scatter half the frozen cinnamon filling in small chunks over the dough. Press remaining dough on top, then scatter remaining filling. Gently fold twice with a spatula to create swirls while keeping chunks visible.

Chill dough: Scoop 2‑tablespoon portions of dough, ensuring both dough and filling are evenly included. Freeze 10 minutes to firm.

Bake: Arrange dough balls on prepared sheets, spaced 2 inches apart. For thicker cookies, bake as is; for thinner cookies, gently press tops before baking. Bake 10–12 minutes until edges are lightly golden and centers are slightly soft.

Cool: Let cookies cool 10–15 minutes on the baking sheet before transferring to a rack.