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

Combine granulated sugar, ground cinnamon, and salt for the cinnamon ripple in a small bowl.

Cream together the unsalted butter and both granulated and light brown sugars until light and fluffy.

Add the large eggs one at a time, beat well after each addition, then mix in sour cream and vanilla extract.

Whisk together all-purpose flour, baking powder, baking soda, and salt in a separate bowl, then fold into the wet mixture.

Spread half of the batter in the prepared pan and sprinkle half of the cinnamon-sugar mixture on top.

Dollop remaining batter on top and spread gently; it doesn't need to be perfectly smooth.

Combine all-purpose flour, granulated sugar, and ground cinnamon for the streusel topping; mix in melted butter until crumbly.

Sprinkle the streusel mixture over the batter and bake for about 50 minutes until golden brown.

Let the cake cool in the pan on a wire rack for at least 15 minutes before slicing and serving.