In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until well combined.

Cream the unsalted butter with both brown and granulated sugars until light and fluffy—about 3-5 minutes.

Add the eggs, one at a time, followed by the vanilla extract. Gradually mix in the dry ingredients until just combined.

Shape the dough into disks, wrap them in plastic, and refrigerate for 1 hour.

In a medium bowl, combine brown sugar, cornstarch, and cinnamon, mixing until evenly distributed.

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

Roll the chilled dough out to about ¼-inch thickness and cut into 3x4 inch rectangles.

Spoon about 1 tablespoon of filling onto the center of one rectangle, place another rectangle on top and seal.

Crimp the edges with a fork and brush the tops lightly with milk.

Bake for 12-15 minutes until light golden and slightly puffed. Cool for 5 minutes on the baking sheet.

Whisk together powdered sugar and milk to achieve a smooth icing. Drizzle over cookies and add sprinkles if desired.