In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.

In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar using an electric mixer on medium speed for 2-3 minutes until light and fluffy.

Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract until fully incorporated.

Gradually add the dry mixture to the wet mixture, mixing on low speed until just combined.

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

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

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

Roll out one disk of dough on a lightly floured surface to about ¼ inch thick.

Cut into rectangles approximately 3×4 inches, aiming for about 12 rectangles from each disk.

Repeat with the second disk of dough.

Place 1 tablespoon of brown sugar filling in the center of each rectangle, top with another rectangle, and press edges together to seal. Crimp edges with a fork.

Brush tops with a little milk and place cookies on prepared baking sheets.

Bake for 12-15 minutes until edges are lightly golden and tops are set.

Cool on baking sheets for 5 minutes before transferring to a wire rack to cool completely.

Whisk together powdered sugar and milk until smooth. Drizzle icing over cooled cookies and add sprinkles if desired.