Prepare the Cinnamon Filling: In a small bowl, combine softened unsalted butter, brown sugar, all-purpose flour, ground cinnamon, and a pinch of salt. Mix well until smooth and creamy.

Make the Sugar Cookie Dough: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. In a large mixing bowl, beat the softened unsalted butter with granulated and brown sugars until light and fluffy. Add in the egg and vanilla extract, mixing until incorporated.

Combine Wet and Dry Ingredients: Gradually add the dry ingredients into the wet mixture, mixing just until the flour is incorporated. Be careful not to overmix; you want your dough smooth yet still thick.

Assemble the Cookies: Divide the dough into two equal halves, flattening one half into the bottom of a bowl. Carefully drop chunks of the chilled cinnamon filling over this layer, then top with the remaining cookie dough.

Chill the Dough: Form the dough into balls, each about 2 tablespoons in size. Place the dough balls on a baking sheet lined with parchment paper, then pop them in the freezer for 10 minutes.

Bake the Cookies: Preheat your oven to 350°F (175°C). Once heated, arrange the dough balls on a lined baking sheet, ensuring they are spaced about 2 inches apart. Bake for 10-12 minutes, or until the edges are set and the centers look slightly undercooked.