Prepare the Cinnamon Sugar Filling : In a small bowl, mix together softened butter, brown sugar, flour, cinnamon, and a pinch of salt until a smooth, spreadable paste forms. Set aside.

Make the Cookie Dough : In a large bowl, beat softened butter and granulated sugar until light and fluffy. Add vanilla extract, the egg, and the egg yolk, beating until well combined. In a separate bowl, whisk together the all-purpose flour, baking powder, cornstarch, and salt. Gradually add the dry ingredients to the wet mixture, mixing until a soft dough forms.

Roll and Fill the Dough : Divide the dough in half for easier handling. Roll out one portion into a rectangle, about ¼ inch thick, on a lightly floured surface. Spread half of the cinnamon sugar filling evenly over the rolled-out dough, leaving a small border around the edges. Starting from one long edge, roll the dough into a tight log. Wrap the log in plastic wrap and repeat with the remaining dough and filling. Refrigerate the logs for at least 1 hour to firm up.

Slice and Bake : Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper. Slice the chilled logs into ¼-inch-thick cookies and place them on the prepared baking sheets, spaced about 2 inches apart. Bake for 10–12 minutes, or until the edges are lightly golden.

Cool and Serve : Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack. Serve warm or store in an airtight container for up to a week.