Preheat oven to 350 degrees F. Grease a 9-inch square baking pan and line with parchment paper, leaving an overhang for easy removal.

In a medium bowl, whisk together flour, brown sugar, and cinnamon. Add cold butter cubes and use your fingertips to rub into the dry ingredients until you have pea-sized clumps with some sandy bits. Set in the fridge while you make the batter.

In a small bowl, whisk together flour, baking powder, and salt.

In a large bowl, beat room temperature butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 3 minutes. Scrape down the bowl as needed.

Beat in the egg and vanilla until fully combined, about 1 minute. The mixture should look smooth and pale.

Add half the flour mixture and mix on low until just combined. Add all the sour cream and mix until incorporated. Add remaining flour and mix until no dry streaks remain - do not overmix.

Spread half the batter into the prepared pan. Sprinkle with half the streusel. Dollop remaining batter on top and spread gently to cover. Sprinkle with remaining streusel, pressing lightly so it adheres. Bake until golden brown and a toothpick inserted in the center comes out clean, 32 to 36 minutes.

Cool in the pan on a wire rack for at least 20 minutes before lifting out using the parchment overhang. Cut into squares and serve warm or at room temperature.