Prep the pans: Line two baking sheets with parchment. Set oven to 350°F (175°C), but you’ll chill the dough before baking.
Make the filling: In a small bowl, mix brown sugar, granulated sugar, cinnamon, melted butter, and a pinch of salt until sandy. Set aside.
Whisk dry ingredients: In a medium bowl, whisk flour, cornstarch, baking powder, baking soda, and salt.
Cream the butter and sugars: Beat softened butter with brown and granulated sugar until light and fluffy, 2–3 minutes.
Add eggs and vanilla: Mix in eggs one at a time, then vanilla.
Scrape the bowl.
Combine: Add dry ingredients to wet on low speed until just combined. Do not overmix.
Swirl the filling: Sprinkle the cinnamon filling over the dough. Fold gently 2–3 times with a spatula to create ribbons. Do not fully blend; you want streaks.
Portion and chill: Scoop 1/4-cup mounds (about 12 large cookies).
Roll into balls, then gently press the tops to flatten slightly. Chill 30–45 minutes.
Bake: Arrange 6 cookies per sheet. Bake 10–12 minutes until edges are set and centers look slightly underdone.
They’ll finish on the pan.
Cool: Let cookies rest on the sheet 10 minutes, then transfer to a rack. Cool completely before icing.
Make the icing: Stir powdered sugar, milk, vanilla, cinnamon, and salt until smooth and thick but pourable. Adjust with a few drops of milk if needed.
Glaze: Spoon icing over the cooled cookies, leaving a small border to mimic Pop-Tart frosting.
Let set 20–30 minutes.