In a large mixing bowl, cream together 1 cup of unsalted butter, 1 cup of granulated sugar, and 1 cup of brown sugar until light and fluffy, about 3-4 minutes. Add in 2 large eggs and 2 teaspoons of vanilla extract, mixing well.

In a separate bowl, whisk together 3 cups of all-purpose flour, 1 teaspoon of baking powder, and 1 teaspoon of cinnamon until thoroughly mixed. Gradually incorporate the dry mixture into the wet ingredients, stirring gently until a soft dough forms.

Mix 1 cup of brown sugar with 1 tablespoon of cinnamon for the filling until combined. Set aside for assembly.

On a floured surface, roll out the dough to about 1/4-inch thickness. Cut into 3-inch squares. Place a teaspoon of filling in the center of half the squares and cover with the other squares, sealing edges with a fork.

Preheat oven to 350°F (175°C). Arrange cookies on a lined baking sheet and bake for 12-15 minutes or until edges are golden.

Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.