01 - Preheat oven to 350°F. Grease a 9-inch square or round baking pan and optionally line it with parchment paper.
02 - Combine brown sugar and ground cinnamon in a small bowl and set aside.
03 - In a bowl, mix flour, brown sugar, cinnamon, and salt. Add cold diced butter and rub with fingers or use a pastry cutter until coarse crumbs form. Refrigerate until needed.
04 - In a large bowl, beat softened butter and granulated sugar together until light and fluffy. Add eggs one at a time, mixing well after each addition, then incorporate vanilla extract.
05 - Whisk together flour, baking powder, baking soda, and salt in a separate bowl.
06 - Alternately add dry ingredients and sour cream to the butter mixture, beginning and ending with the dry ingredients. Mix gently until just combined.
07 - Spread half of the batter evenly into the prepared pan. Sprinkle the cinnamon-sugar swirl mixture evenly over the batter, then top with remaining batter and smooth the surface.
08 - Evenly distribute the streusel crumb topping over the surface.
09 - Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.
10 - Allow to cool in the pan for 15 minutes, then transfer to a wire rack. Serve warm or at room temperature.