Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a small bowl, combine the softened butter, brown sugar, flour, cinnamon, and salt to make the cinnamon sugar filling. Mix until fully combined and freeze for about 10 minutes.

In a large bowl, beat the softened butter with granulated sugar until light, fluffy, and creamy, about 2-3 minutes.

Add vanilla extract, whole egg, and egg yolk to the butter-sugar mixture and beat until smooth.

In another bowl, whisk together the flour, baking powder, cornstarch, and salt.

Gradually add the dry ingredients to the wet mixture and mix until just combined.

Remove half of the dough and set it aside. Spread the remaining half in the bottom of the mixing bowl.

Scatter the chilled cinnamon sugar filling over the dough layer and top with the reserved half of the dough, pressing it down.

Fold the layered dough and cinnamon sugar filling gently to maintain distinct chunks and swirls.

Scoop out two-tablespoon-sized dough balls and freeze them for 10 minutes.

Arrange the chilled dough balls on baking sheets and bake for 10-12 minutes until the edges begin to brown. Transfer to wire racks to cool completely.