In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set this dry mixture aside.

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

Incorporate the eggs one at a time, ensuring each is fully mixed in before adding the next. Stir in the vanilla extract until evenly combined.

Gradually add the dry mixture to the wet mixture and mix on low speed until just combined—do not overmix.

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

While the dough is cooling, prepare the filling by mixing the brown sugar, cornstarch, and cinnamon in a small bowl. Set it aside.

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 on a lightly floured surface and cut into rectangles (approximately 3x4 inches) to create about 12 rectangles.

Place about 1 tablespoon of the brown sugar filling in the center of one rectangle, cover with another rectangle, and press the edges together to seal. Crimp the edges with a fork for an appealing finish.

Repeat until all cookies are assembled, leaving space between each on the baking sheets.

Brush the tops lightly with milk before baking for 12-15 minutes, until golden at the edges and set on top.

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

Make the icing by whisking together the powdered sugar and milk until smooth. Drizzle this over the cooled cookies and decorate with sprinkles if desired.