In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, and ½ teaspoon of salt. In another bowl, cream ¾ cup of unsalted butter with ¾ cup brown sugar and ¼ cup granulated sugar until light and fluffy, about 2-3 minutes. Add 1 large egg and 1 teaspoon of vanilla extract, mixing well. Gradually combine the dry ingredients with the wet until just mixed; avoid overmixing to keep your Brown Sugar Pop Tart Cookies tender.

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

In a small mixing bowl, combine ½ cup of brown sugar, 1 tablespoon of cornstarch, and 1 teaspoon of cinnamon. Mix well to ensure that no lumps remain.

Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper. Take one chilled disk of dough and roll it out on a floured surface to about ¼ inch thick. Using a knife or pizza cutter, cut the rolled dough into rectangles approximately 3x4 inches. Repeat with the second dough disk.

On half of the rectangles, place about 1 tablespoon of the prepared filling in the center. Top each filled rectangle with another rectangle of dough and gently press the edges together to seal. Use a fork to crimp the edges and brush the tops lightly with milk.

Carefully place the assembled cookies on the prepared baking sheets, leaving space between them. Bake for 12-15 minutes, or until the edges are lightly golden. Allow to cool for 5 minutes before transferring to a wire rack.

In a medium bowl, whisk together 1 cup of powdered sugar and 1-2 tablespoons of milk until smooth. Adjust the milk for desired consistency.

Drizzle the icing over the cooled cookies and add sprinkles if desired. Let set before serving.