Step 1: In a medium bowl, whisk together the all-purpose flour, baking powder, 1 teaspoon ground cinnamon, and salt. Set aside.
Step 2: In a large bowl, using an electric mixer, cream together the softened unsalted butter, granulated sugar, and ½ cup packed brown sugar until light and fluffy.
Step 3: Beat in the large egg and vanilla extract until well combined.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a soft dough forms. Do not overmix.
Step 5: Turn the dough out onto a lightly floured surface. Form it into a rough rectangle, then roll it out to a rectangle approximately 12x10 inches in size.
Step 6: Prepare the filling by combining the remaining ½ cup packed brown sugar and 2 teaspoons ground cinnamon in a small bowl. Sprinkle this mixture evenly over the rolled-out dough, leaving a small border (about ½ inch) along one of the long edges.
Step 7: Starting from the long edge opposite the border, carefully and tightly roll the dough into a log. Wrap the log tightly in plastic wrap and chill in the refrigerator for at least 1-2 hours, or until firm. This helps maintain the swirl shape during baking.
Step 8: Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper. Once the dough log is firm, unwrap it and slice it into ½-inch thick rounds using a sharp knife. Place the cookies about 2 inches apart on the prepared baking sheet.
Step 9: Bake for 10-12 minutes, or until the edges are lightly golden and the centers are set but still look soft. Do not overbake. Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.