Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs, one at a time, mixing well after each addition.

Stir in the sour cream and vanilla extract until combined.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until just combined. Do not overmix.

In another bowl, prepare the streusel topping by combining the brown sugar, flour, and cinnamon. Stir in the melted butter until the mixture resembles coarse crumbs.

Pour half of the batter into the prepared baking pan and spread evenly. Sprinkle half of the streusel topping over the batter.

Pour the remaining batter on top and finish with the rest of the streusel.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.