Begin by whisking together 2 cups of all-purpose flour, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/4 teaspoon of salt in a large bowl.

In a separate bowl, cream 1/2 cup unsalted butter, 1 cup packed brown sugar, and 1/2 cup granulated sugar until the mixture is light and fluffy.

Incorporate 2 room temperature eggs one at a time, mixing well after each addition, and stir in 1 teaspoon vanilla extract.

Gradually add the dry ingredients to the wet mixture until just combined. Divide the dough into two disks, wrap them in plastic wrap, and chill in the refrigerator for at least 1 hour.

While the dough chills, in a medium bowl, combine 1/2 cup brown sugar, 2 tablespoons cornstarch, and 1 teaspoon cinnamon. Mix until evenly blended.

Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper. Roll out one chilled dough disk to about 1/4 inch thick and cut into rectangles measuring 3x4 inches.

Take one rectangle of dough and place about 1 tablespoon of the prepared filling in the center. Position another rectangle on top, sealing edges and crimping with a fork.

Brush the tops with milk and bake the assembled cookies for 12-15 minutes or until golden brown. Cool on the baking sheet for 5 minutes before transferring to a wire rack.

Whisk together 1 cup of powdered sugar and 2-3 tablespoons of milk until smooth and glossy. Drizzle over cooled cookies and finish with sprinkles if desired.