Preheat your oven to 350°F (175°C) and grease a 9x9-inch metal pan with unsalted butter.

In a small bowl, whisk together granulated sugar, cinnamon, and a pinch of salt for the cinnamon swirl filling.

In a large mixing bowl, cream softened unsalted butter with both sugars until light and fluffy (3-5 minutes).

Beat in the eggs one at a time, blending well after each addition to achieve a smooth batter.

Stir in the sour cream and vanilla extract until combined and smooth.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and a pinch of salt.

Gradually fold the dry ingredients into the wet mixture until just combined.

Spread slightly less than half of the batter into the prepared pan and sprinkle with the cinnamon mixture.

Top with the remaining batter, smoothing it out evenly.

In a small bowl, mix together streusel topping ingredients until crumbly and sprinkle evenly over the batter.

Bake for 50 minutes, or until a toothpick comes out clean or with a few moist crumbs.

Cool in the pan for 15-20 minutes, then cut into squares to serve warm or at room temperature.