Cinnamon sugar filling:

• 4 tablespoons unsalted butter, softened
• ⅓ cup brown sugar, packed
• 1½ tablespoons all-purpose flour
• 2½ teaspoons ground cinnamon
• pinch of salt

Cookies:

• 1 cup unsalted butter, softened but still cool to the touch
• 1 ⅓ cups granulated sugar
• 1 ¾ teaspoons pure vanilla extract
• 1 large egg
• 1 large egg yolk
• 2 ¾ cups all-purpose flour
• ¾ teaspoon baking powder
• 1 teaspoon cornstarch
• ¼ teaspoon salt