Preheat oven to 350°F and grease an 8-inch square pan, lining it with parchment paper.
Prepare the topping by combining brown sugar, 1/2 cup flour, and 1 1/2 tsp cinnamon in a bowl; cut in 6 tbsp cold cubed butter until crumbly and refrigerate.
In a medium bowl, whisk together remaining flour, baking powder, baking soda, and salt.
In a large bowl, cream together the softened butter and granulated sugar until smooth and fluffy.
Add eggs and vanilla extract to the butter mixture, beating well.
Mix in the sour cream until combined.
Gradually add the dry ingredients to the wet mixture, alternating with the milk, stirring until just combined.
Spread half the batter into the prepared pan and sprinkle with half the streusel topping.
Top with the remaining batter and the rest of the streusel.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.