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

In a large mixing bowl, cream the softened unsalted butter with both brown sugar and granulated sugar until light and fluffy, about 3-4 minutes.

Add the eggs, one at a time, and mix well after each addition before stirring in the vanilla extract.

Gradually mix the dry ingredients into the wet ingredients on low speed until just combined.

Divide the dough in half and shape each half into a disk. Wrap each disk tightly in plastic wrap and refrigerate for at least 1 hour.

In a small bowl, combine the brown sugar, cornstarch, and cinnamon, mixing until well blended.

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

Remove the dough from the fridge and roll out one disk on a floured surface to about ¼ inch thick. Cut the dough into rectangles approximately 3x4 inches.

On half of the dough rectangles, generously place about 1 tablespoon of the prepared filling in the center. Place another rectangle on top.

Press gently around the edges to seal and crimp the edges tightly with a fork.

Brush a little milk on top for a nice golden color.

Bake for approximately 12-15 minutes until lightly golden. Let them cool for 5 minutes before transferring them to wire racks.

Whisk together the powdered sugar and milk in a small bowl until smooth. Drizzle over cooled cookies and add sprinkles if desired.

Serve warm or at room temperature and store any leftovers in an airtight container for up to a week.