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

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

Add eggs, one at a time, mixing well after each addition, then stir in vanilla extract.

Gradually blend the dry ingredients into the wet mixture until just combined.

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

While the dough chills, prepare the filling by mixing brown sugar, cornstarch, and optional cinnamon in a small bowl.

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

Unwrap one of the dough disks and roll it out on a lightly floured surface into a rectangle about ¼ inch thick.

Cut this rolled dough into 3x4 inch rectangles using a sharp knife or pizza cutter.

Place a teaspoon of the filling onto the center of half of the dough rectangles.

Cover each filled rectangle with another dough piece, pressing down gently around the edges to seal.

Use a fork to crimp the edges securely and brush the tops of each cookie with a splash of milk.

Transfer the assembled cookies to the preheated oven and bake for 12-15 minutes.

Allow the cookies to cool on the baking sheets for a few minutes before transferring them to wire racks to cool completely.

In a medium bowl, whisk together powdered sugar and 2 tablespoons of milk until smooth.

Drizzle this icing over the cooled Brown Sugar Pop Tart Cookies, optionally sprinkle with colorful sprinkles.