In a small bowl, mix together brown sugar, all-purpose flour, and cinnamon. Add the cold, diced unsalted butter and use your fingers to combine until coarse crumbs form. Set aside.

Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.

In a large mixing bowl, beat the softened unsalted butter and granulated sugar with an electric mixer until light and fluffy, about 3-4 minutes.

Add the large eggs one at a time and mix well, then add the vanilla extract and scrape down the sides of the bowl.

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

Gradually add the dry ingredients to the creamed mixture, alternating with sour cream. Begin and end with the dry mixture, mixing until just combined.

Gently fold in the blueberries with a spatula.

Spread the batter into the prepared baking dish, smoothing the top with a spatula.

Evenly sprinkle the crumb topping over the batter.

Bake in the preheated oven for 40-45 minutes or until a toothpick comes out clean.

Allow the cake to cool in the pan for about 15 minutes before slicing.