Prepare the Cinnamon Sugar Filling by combining unsalted butter, brown sugar, flour, cinnamon, and a pinch of salt in a mixing bowl. Blend until smooth yet slightly chunky, then chill in the freezer for 10 minutes.

Make the Cookie Dough by creaming together softened unsalted butter and granulated sugar using an electric mixer until light and fluffy. Add vanilla extract, whole egg, and yolk, beating until fully combined.

Combine dry ingredients by whisking together all-purpose flour, baking powder, cornstarch, and salt in a separate bowl. Gradually add this to the wet mixture until just incorporated.

Layer the Dough by dividing the prepared dough into two halves. Spread one half in the bottom of a bowl, layer with half the chilled cinnamon sugar filling, cover with the second half of dough, and sprinkle remaining filling on top. Gently fold to create swirls without overmixing.

Shape and Chill portions of dough into balls and place on a parchment-lined baking sheet. Freeze for 10 minutes to prevent spreading during baking.

Bake preheated oven to 350°F (180°C) for 10-12 minutes until edges are golden brown, but centers remain soft. Remove from oven when ready.

Cool for 5 minutes on baking sheets before transferring to a wire rack. Dust with powdered sugar or drizzle with icing if desired before serving.