In a bowl, add the room temperature butter, granulated sugar, and brown sugar. Beat on medium-high speed for about 2 to 3 minutes until the mixture looks light and fluffy.

Add the vanilla extract and eggs to the butter-sugar mixture. Beat again for another 1 to 2 minutes until everything is combined well.

In a separate bowl, whisk together the dry ingredients: 3 3/4 cups of cake flour, cornstarch, baking soda, baking powder, and kosher salt. Blend well without lumps.

Add the dry ingredients to the wet mixture little by little, mixing for about 1 to 2 minutes. Scrape down the sides and bottom of the bowl to ensure even mixing.

Cover the dough and place it in the fridge for one hour.

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

In a bowl, combine the filling ingredients: 5 tablespoons softened butter, 3/4 cup packed light brown sugar, 1 teaspoon cinnamon, and 2 tablespoons of cake flour. Stir until smooth.

Roll heaping teaspoons of the filling into balls and place them on one lined baking sheet.

Scoop out the chilled dough with a 1/4 cup measuring cup, creating a well in one half. Place the filling ball inside and cover it with the other half. Pinch the seams to seal.

Arrange the cookie balls on the baking sheets, leaving about 2 inches between them. Bake for 11 to 13 minutes until the tops are set and edges lightly golden.

Allow the cookies to cool on the baking sheet for about 5 minutes, then transfer to a wire rack.

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

Spoon the icing generously on top of each cookie, allowing it to spread. Let icing set for about 15 minutes before serving.