Prepare the Cinnamon Filling: In a small mixing bowl, combine softened butter, brown sugar, flour, cinnamon, and a pinch of salt. Mix until smooth and well-blended.

Make the Sugar Cookie Dough: In a large bowl, whisk together all-purpose flour, baking powder, baking soda, and salt until combined. In a separate bowl, cream the softened unsalted butter with granulated and brown sugars until light and fluffy. Mix in the egg and vanilla extract, then gradually incorporate the dry ingredients until a soft dough forms.

Assemble the Cookies: Divide the cookie dough into halves. Press one half into the bottom of a mixing bowl to form an even layer. Remove the frozen cinnamon filling from the freezer and cut it into small pieces, distributing it evenly over the dough layer. Cover with the remaining dough, gently folding to create a marbled effect.

Chill the Dough: Portion the dough into 2-tablespoon balls and place them on a parchment-lined baking sheet. Cover with plastic wrap and freeze for approximately 10 minutes.

Bake: Preheat your oven to 350°F (175°C). Arrange the chilled dough balls on a parchment-lined baking tray. Bake for 10-12 minutes, until edges are set and centers are slightly undercooked.

Cool on a Wire Rack: Allow cookies to cool on the tray for 5 minutes, then transfer to a wire rack to cool completely.