In a large mixing bowl, whisk together flour, baking powder, baking soda, and salt.

In a separate bowl, cream the unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.

Beat in the eggs one at a time, followed by the vanilla extract.

Gradually mix in the dry ingredients until just combined. Divide the dough into two portions, shape into disks, wrap in plastic, and chill for at least 1 hour.

While the dough chills, prepare the filling by mixing brown sugar, cornstarch, and cinnamon in a bowl.

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

Roll out one disk of chilled dough to about ¼ inch thickness and cut into 3x4 inch rectangles.

Place about 1 tablespoon of filling in the center of half the dough rectangles. Cover with another rectangle and press down lightly around the edges.

Brush the tops of the cookies with milk, then bake for 12-15 minutes until lightly golden.

Let them cool for 5 minutes before transferring to a cooling rack.

In a small bowl, mix powdered sugar with milk until smooth, then drizzle over cooled cookies and add sprinkles if desired.