1-Preheat oven to 350°F with racks near the center and line two baking sheets with parchment paper to prevent sticking.

2-Mix the cinnamon sugar filling ingredients together until well combined, then freeze for about 10 minutes to make it easier to handle.

3-In a large bowl, beat the butter and sugar until light and creamy, then add the vanilla extract, egg, and egg yolk; mix until everything is fully combined for a smooth base.

4-In a separate bowl, combine the flour, baking powder, cornstarch, and salt; gradually add this dry mixture to the wet ingredients and mix until the dough comes together nicely.

5-Remove half the dough and spread the remaining dough in the bottom of the mixing bowl, then add half the cinnamon filling in small pieces evenly on top for that swirl effect.

6-Spread the reserved dough over the cinnamon mixture and add the remaining cinnamon filling evenly on top, building layers for the best flavor distribution.

7-Fold and mix the dough briefly by hand to create swirls and chunks of cinnamon mixture, being gentle to keep those pockets intact.

8-Using a cookie scoop, portion the dough into two-tablespoon scoops, making sure the cinnamon is well mixed in each one, then freeze the dough balls for 10 minutes to help them hold shape.

9-Place the dough balls on the parchment-lined baking sheets, spacing them a couple of inches apart; leave them in mounds for thicker cookies or pat them down slightly for thinner ones, and refrigerate any extra portions not baked right away.

10-Bake for 10-12 minutes until the edges brown slightly and the centers stay a bit underdone, then cool on wire racks to let them set properly.

11-For round cookies, right after baking, gently swirl a round glass or cutter on the edges while warm, or use the back of a spoon to push the edges inward for a clean shape that mimics classic cinnamon rolls.