Make the dough: In a large bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy, about 3 minutes. Beat in eggs one at a time, then add vanilla extract.

In a separate bowl, whisk together cake flour, cornstarch, baking powder, baking soda, and salt. Gradually add dry ingredients to wet ingredients, mixing just until combined. Cover and refrigerate dough for at least 1 hour.

Make the filling: In a medium bowl, mix brown sugar and cinnamon. Add melted butter and stir until mixture resembles wet sand. Roll filling into teaspoon-sized balls and refrigerate until firm.

Assemble cookies: Preheat oven to 350°F and line baking sheets with parchment paper. Scoop 2 tablespoons of chilled dough, flatten in your palm, place a filling ball in the center, then wrap dough around filling and seal completely. Roll into a smooth ball.

Place dough balls 2 inches apart on prepared baking sheets. Bake for 11-13 minutes until edges are set and lightly golden. Let cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Make the glaze: Whisk together powdered sugar, cinnamon, melted butter, and milk until smooth. Place parchment paper under cooling rack and spoon glaze over each cookie. Let set for 15 minutes before serving.