In a large mixing bowl, cream together the unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 3-5 minutes. Mix in the eggs and vanilla extract until fully combined. Gradually add the flour, baking powder, baking soda, and salt, mixing until a soft dough forms.

In a separate bowl, combine the brown sugar and ground cinnamon until blended. Gradually stir in milk until you achieve a thick paste. Set aside.

Flour your countertop and roll out the cookie dough to 1/4 inch thick. Cut into desired shapes, placing half onto a lined baking sheet. Spoon filling onto each and cover with another cutout. Seal edges with a fork.

Preheat oven to 350°F (175°C). Bake cookies for 12-15 minutes until golden brown. Monitor closely for best results.

Let cookies cool on a wire rack. Mix powdered sugar with milk to create icing and drizzle over cooled cookies. Add sprinkles if desired.