Make the Cookie Dough:

• In a large bowl, whisk together the flour, baking powder, and salt. Set aside.
• In a separate bowl, cream together the softened butter and granulated sugar until light and fluffy.
• Add the egg and vanilla extract and mix until well combined.
• Gradually add the dry ingredients to the wet ingredients, mixing until the dough comes together. Add milk, a little at a time, to achieve a soft, rollable dough.

Prepare the Brown Sugar Filling:

• In a small bowl, combine the brown sugar, cornstarch, cinnamon, and salt.
• Stir in the melted butter to form a thick paste. Set aside.

Assemble the Cookies:

• Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
• Roll out the dough on a lightly floured surface to about ¼ inch thick. Use a cookie cutter or a knife to cut the dough into rectangles or squares.
• Place half of the dough pieces on the prepared baking sheet. Spoon a small amount of the brown sugar filling in the center of each rectangle, being careful not to overfill.
• Top each with a second piece of dough, gently pressing the edges to seal the filling inside. Use a fork to crimp the edges and create a decorative seal.

Bake the Cookies:

• Bake the cookies in the preheated oven for 12-15 minutes, or until the edges are golden brown and the cookies are crisp.

Glaze the Cookies (Optional):

• In a small bowl, whisk together powdered sugar, milk, and vanilla extract until smooth. If the glaze is too thick, add a little more milk until it reaches a drizzling consistency.
• Once the cookies are cool, drizzle the glaze over the top of each cookie. Add sprinkles for extra fun, if desired.