In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well combined.

In a large mixing bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy.

Add the egg and vanilla extract, mixing well until fully incorporated.

Gradually add the dry ingredients to the butter-sugar mixture, mixing on low speed until just combined.

Divide the dough in half, shape each portion into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour.

Prepare the filling by mixing brown sugar, cornstarch, and cinnamon in a small bowl.

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

Roll out one disk of dough to about ¼ inch thick and cut out rectangles.

Place filling in the center of half the rectangles, top with another rectangle, and seal the edges.

Brush the tops with milk and bake for 12-15 minutes or until lightly golden.

Let cookies cool on sheets for 5 minutes, then transfer to a wire rack.

Whisk powdered sugar and milk until smooth, and drizzle icing over the cooled cookies.