In a medium bowl, combine softened butter, brown sugar, flour, cinnamon, and a pinch of salt. Mix until smooth and creamy, ensuring there are no lumps. Transfer the mixture to the freezer for 10 minutes to firm up.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until evenly mixed. In a large mixing bowl, beat the softened butter and both sugars together until light and fluffy, about 2-3 minutes. Add in the egg and vanilla extract, mixing until well incorporated, then gradually blend in the dry ingredients.

Divide the sugar cookie dough into two equal portions. Flatten one half into a rectangle on parchment paper and spread the chilled cinnamon filling over the surface evenly. Layer the other half of the dough gently over the cinnamon filling, creating swirls without overmixing.

Once assembled, portion the dough into balls, approximately 2 tablespoons each, and place them on a baking sheet, spaced apart. Pop the baking sheet in the freezer and chill the dough balls for about 10 minutes.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Arrange the chilled dough balls on the sheet, placing them about 2 inches apart. Bake for 10-12 minutes, until the edges are set and the centers are slightly undercooked.

Once baked, remove the cookies from the oven and cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.