Brown Sugar Pop Tart Cookies

Heat oven to 350°F and line 2 baking sheets with parchment paper.
Make the filling: In a small bowl, stir together brown sugar, cinnamon, and flour. Pour in the melted butter and mix until it looks like damp sand. Set aside.
Make the dough: In a large bowl, beat the softened butter, brown sugar, and granulated sugar on medium-high until creamy and light, 2–3 minutes.
Beat in the egg, egg yolk, and vanilla until smooth, scraping the bowl as needed.
In a separate bowl, whisk the flour, cornstarch, baking soda, salt, and cinnamon. Add to the wet ingredients and mix on low just until the flour disappears. If the dough feels very soft, chill 15–20 minutes.
Portion the dough into 12 equal mounds (about 3 tablespoons each). Flatten each into a thick disc in your palm.
Spoon about 1 to 1 1/2 teaspoons of the cinnamon filling into the center of each disc. Wrap the dough around the filling and pinch to seal. Place seam-side down on the prepared sheets, spacing well.
Bake 10–12 minutes, until the edges are set and the tops look just dry but still pale. Let the cookies cool on the sheet 10 minutes, then transfer to a rack to cool completely.
Make the glaze: Whisk powdered sugar, cinnamon, vanilla, and 2 tablespoons milk until smooth. Add more milk a few drops at a time to reach a thick, pourable drizzle.
Drizzle the cooled cookies with the cinnamon glaze. Let it set before serving.