Preheat your oven to 180°C (350°F) and grease an 8-inch square baking pan with butter or cooking spray, lining the bottom with parchment paper.

In a small bowl, combine ¾ cup of all-purpose flour, ½ cup of light brown sugar, ½ teaspoon of ground cinnamon, and a dash of salt. Mix with melted unsalted butter until crumbly.

In a large mixing bowl, beat together ½ cup of unsalted butter and ¾ cup of granulated sugar until light and fluffy, about 2-3 minutes.

Add 1 large egg, 1 egg yolk, ½ cup of sour cream, and 1 teaspoon of vanilla extract to the butter-sugar mixture, mixing until smooth.

Sift together 1 ¼ cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt. Fold this into the wet ingredients.

Pour the batter into the prepared baking pan, smoothing the top, then sprinkle the streusel topping over the batter.

Bake for approximately 30 minutes, until golden brown and a toothpick inserted comes out clean.

Allow the cake to cool completely in the pan on a wire rack, then lift it out using the parchment overhang and cut into squares.