Make the Dough:
Cream the butter, granulated sugar, and brown sugar together for 2–3 minutes. Add eggs and vanilla; beat until light and fluffy.
In a separate bowl, whisk together cake flour, cornstarch, baking soda, baking powder, and salt.
Gradually mix the dry ingredients into the wet until fully combined.
Cover and chill the dough for 1 hour.

Make the Filling:
Mix butter, brown sugar, cinnamon, and flour until smooth.
Scoop out 2-teaspoon-sized portions, roll into balls, and place on a plate or baking sheet.

Assemble the Cookies:
Scoop about ¼ cup of cookie dough, split it in half, and press a well into one half.
Insert one filling ball, cover with the other half, and seal the edges. Roll into a ball.
Place on lined baking sheets about 2 inches apart.

Bake:
Preheat oven to 350°F (175°C).
Bake for 11–13 minutes until the edges are golden and the tops are just set.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack.

Make the Glaze:
Whisk powdered sugar, cinnamon, melted butter, and milk until smooth.
Spoon the glaze over each cookie and allow to set for 15 minutes before serving.