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

In a separate large bowl, cream the softened unsalted butter, brown sugar, and granulated sugar with a hand mixer until light and fluffy.

Add in the eggs and vanilla extract, mixing until smooth.

Gradually incorporate the dry ingredients until just combined. Divide the dough into two disks, wrap them in plastic, and refrigerate for at least 1 hour to firm up.

In a small bowl, mix together the brown sugar, cornstarch, and cinnamon for the filling. Set aside.

Preheat your oven to 350°F (175°C). Roll out one disk of the chilled dough to a thickness of about ¼ inch. Trim into 3x4 inch rectangles.

Take one rectangle of dough and place about 1 tablespoon of your filling in the center. Seal with another rectangle on top and crimp edges with a fork.

Brush tops with milk and arrange on a baking sheet. Bake for 12-15 minutes until golden.

Cool slightly on baking sheet, then transfer to a wire rack.

Whisk together powdered sugar and milk for icing. Drizzle over cookies once cooled.