Preheat the oven to 350°F (175°C). Grease a 9x9-inch baking pan with butter or line with parchment paper.

Make the streusel: In a medium bowl, whisk together the brown sugar, 1/2 cup flour, and cinnamon. Add the cold butter cubes. Use your fingertips or a pastry cutter to work the butter into the flour mixture until it forms clumps ranging from pea-sized to small pebbles. Don't overmix—some larger clumps are good. Set aside in the refrigerator.

Make the cake: In a small bowl, whisk together 1 1/2 cups flour, baking soda, baking powder, and salt.

In a large bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides.

Add the eggs one at a time, beating well after each addition. Beat in the vanilla.

With the mixer on low, add half of the flour mixture, then the sour cream, then the remaining flour mixture. Mix just until combined—stop as soon as you see no more streaks of flour. Do not overmix or the cake will be tough.

Spread the batter evenly into the prepared pan. It will be thick. Sprinkle the streusel topping evenly over the batter, covering every bit.

Bake for 35-40 minutes, until a toothpick inserted into the center comes out clean or with a few moist crumbs. The top should be golden brown and the streusel will look set. If the topping is browning too quickly after 20 minutes, tent loosely with foil.

Cool in the pan on a wire rack for at least 20 minutes before slicing. Serve warm or at room temperature.