In a mixing bowl, add the room-temperature butter, granulated sugar, and brown sugar. Beat on medium-high speed for 2-3 minutes until creamy.

Add the vanilla extract and eggs to the mixture. Beat for another 1-2 minutes until it becomes light and fluffy.

In a separate bowl, whisk together the dry ingredients: cake flour, cornstarch, baking soda, baking powder, and kosher salt.

Gradually combine the dry ingredients with the wet mixture. Mix on medium speed for 1-2 minutes, scraping the sides and bottom of the bowl as needed.

Cover the dough and chill it in the refrigerator for one hour.

Preheat your oven to 350°F and prepare two large baking sheets with parchment paper.

For the filling, mix together the softened butter, brown sugar, cinnamon, and cake flour in a medium bowl until smooth.

Scoop about 2 teaspoons of the filling, roll it into a ball, and place it on the prepared baking sheet.

For the cookies, use a 1/4 cup measuring cup to scoop the chilled dough. Divide it in half and create a well in one half. Place one ball of filling in the well, cover it with the other half, and pinch the edges to seal it. Roll it into a ball.

Place the cookie dough balls on the baking sheet, spaced about 2 inches apart. Bake for 11-13 minutes or until the tops are set with lightly golden edges.

Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

While cooling, prepare the icing by whisking together the powdered sugar, melted butter, milk, and cinnamon until smooth.

Spoon the icing over each cookie once cooled, allowing it to spread to the edges. Let the icing set for 15 minutes and enjoy!