01 - Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, ensuring an overhang for easy lifting.
02 - In a medium bowl, combine flour, brown sugar, cinnamon, and salt. Stir in melted butter until the mixture forms moist crumbs. Set aside.
03 - In a small bowl, combine brown sugar and cinnamon for the swirl. Set aside.
04 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt for the cake base.
05 - In a large bowl, cream softened butter with granulated sugar until the mixture is light and fluffy, approximately 2 minutes. Beat in eggs one at a time, followed by vanilla extract.
06 - Gradually add the flour mixture to the wet ingredients in two additions, alternating with the sour cream. Mix gently until just combined.
07 - Spread half of the batter evenly into the prepared pan. Sprinkle the cinnamon swirl mixture evenly over the batter. Gently spread the remaining batter on top.
08 - Sprinkle the streusel topping evenly over the final batter layer.
09 - Bake for 32–36 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
10 - Allow the bars to cool completely in the pan on a wire rack. Lift out using the parchment overhang, slice into bars, and serve.