Prepare the cookie dough:
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy (about 2-3 minutes). Add the egg and vanilla extract, and continue to beat until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. If the dough seems too dry, add a tablespoon of milk at a time until it reaches a smooth, slightly stiff consistency.
Divide the dough into two balls, wrap them in plastic wrap, and chill them in the refrigerator for at least 1 hour.

Make the brown sugar cinnamon filling:
In a small bowl, combine the brown sugar, cinnamon, softened butter, and flour. Stir until the mixture forms a thick paste. Set aside.

Shape and assemble the cookies:
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Roll out one ball of chilled dough on a floured surface to about ¼ inch thickness. Using a cookie cutter (or knife), cut the dough into rectangles or any shape you like, such as castle towers or castles.
Place a small amount of the brown sugar cinnamon filling in the center of half of the shapes. Place a second piece of dough on top, pressing the edges to seal them. You can use a fork to crimp the edges for a decorative touch.
Repeat with the remaining dough and filling.

Bake the cookies:
Place the shaped cookies on the prepared baking sheet and bake for 12-15 minutes, or until the edges are golden brown and the centers are set. Allow them to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

Glaze and decorate:
If you’d like to add a glaze, whisk together powdered sugar, milk, and vanilla extract in a small bowl until smooth. Drizzle the glaze over the cooled cookies and decorate with sprinkles if desired.