In a large mixing bowl, cream together 1 cup of softened unsalted butter, 1 cup of packed brown sugar, and 1/2 cup of granulated sugar until fluffy.

Beat in 2 large eggs and 1 teaspoon of vanilla extract until fully combined.

In a separate bowl, whisk together 3 cups of all-purpose flour, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt.

Gradually add the dry mixture to the wet ingredients, mixing until just combined. Chill the dough in the refrigerator for about 30 minutes.

While the dough chills, mix together 1 cup of brown sugar and 1 tablespoon of cinnamon for the filling.

Remove half of the chilled dough and roll it out to 1/4 inch thickness, cutting into squares or rectangles, about 2-3 inches.

Place a spoonful of the cinnamon-brown sugar filling on half of the squares. Top each with another dough square and use a fork to seal the edges.

Preheat oven to 350°F (175°C). Bake cookies for 12-15 minutes until the edges are lightly golden.

Let cool for 10 minutes, then prepare icing by whisking together 1 cup of powdered sugar and 1-2 tablespoons of milk. Drizzle over cookies and add sprinkles if desired.