In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. This will be our dry mixture.

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. Be careful not to overmix.

Once the dough is formed, divide it into two equal portions. Shape each portion into a disk, wrap them in plastic wrap, and refrigerate for at least 1 hour.

In a small bowl, combine the brown sugar, cornstarch, and cinnamon. Mix well until evenly distributed.

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

Take one disk of dough out of the refrigerator and roll it out to about ¼ inch thick on a lightly floured surface.

Cut the rolled-out dough into rectangles approximately 3×4 inches, aiming for about 12 rectangles from each disk.

Place about 1 tablespoon of the brown sugar filling in the center of each rectangle.

Cover each filled rectangle with another rectangle of dough. Press the edges together to seal, crimping with a fork for decoration.

Repeat until all rectangles are filled and assembled, placing on prepared baking sheets with space between.

Brush the tops of cookies with milk.

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

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

In a small bowl, whisk together the powdered sugar and 2 tablespoons of milk until smooth. Adjust consistency with more milk if necessary.

Drizzle icing over cooled cookies and add sprinkles if desired.