In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. In a large mixing bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes. Beat in the egg and vanilla extract, then gradually mix in the dry ingredients until just combined. Chill shaped dough disks in the refrigerator for at least 1 hour.

Prepare the filling by mixing the brown sugar, cornstarch, and cinnamon in a small bowl. Stir until thoroughly combined and set aside.

Preheat your oven to 350°F (175°C). On a lightly floured surface, roll the chilled dough to about 1/4 inch thick. Cut out 3x4 inch rectangles—two for each cookie.

Place half of the rectangles on a parchment-lined baking sheet. Spoon about 1 tablespoon of filling onto the center of each rectangle. Top with another rectangle, sealing edges with a fork.

Bake for 12-15 minutes or until lightly golden. Allow to cool on the baking sheet for 5 minutes before transferring to a wire rack.

While cookies cool, whisk together powdered sugar and milk until smooth. Drizzle icing over cookies and add sprinkles if desired.

Let the icing set before serving. These cookies are great warm or at room temperature.