Cream together 1 cup of softened unsalted butter, 1 cup of packed brown sugar, and 1/2 cup of granulated sugar until light and fluffy, about 2-3 minutes.

Add 2 large eggs and 1 teaspoon of vanilla extract, blending until thoroughly combined.

Gradually mix in 3 cups of all-purpose flour, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt to form a cohesive dough.

Combine 1 cup of brown sugar and 1 tablespoon of ground cinnamon in a separate bowl, mixing well until evenly blended.

Lightly flour your work surface and roll out the dough to about 1/4-inch thickness.

Slice the dough into rectangles, about 3x4 inches, ensuring even sizes.

Place a teaspoon of the cinnamon-brown sugar filling onto the center of half of the rectangles.

Top with the remaining rectangles, pressing down gently around the edges, and seal tightly with a fork.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Arrange cookies about 2 inches apart and bake for 12-15 minutes, or until golden brown.

Let the cookies cool on the baking sheet for about 5 minutes before transferring to a wire rack.

Prepare icing by mixing 1 cup of powdered sugar with 1-2 tablespoons of milk until smooth, and drizzle over cooled cookies.