In a mixing bowl, whisk together flour, baking powder, baking soda, and salt until well combined. In a separate bowl, cream butter with brown sugar and granulated sugar until fluffy. Add the egg and vanilla extract, mixing until smooth. Gradually add the dry mixture to form a soft dough.

Divide the prepared dough into two equal disks and wrap each in plastic wrap. Refrigerate for at least 1 hour.

In a medium bowl, combine brown sugar, cornstarch, and cinnamon. Mix well to create a sweet filling.

Preheat your oven to 350°F (175°C). Roll out one disk of dough on a floured surface to about ¼ inch thick and cut into rectangles.

Place approximately 1 tablespoon of filling in the center of half the rectangles. Cover each with another dough piece and crimp the edges.

Transfer to a lined baking sheet, brush with milk, and bake for 12-15 minutes or until golden brown.

Mix powdered sugar with milk until smooth for icing. Drizzle over cooled cookies and add sprinkles if desired.