Preheat your oven to 350°F. Grease a 9-inch round or square baking pan, then dust it with flour, or line it with parchment paper for easy removal.

In a small bowl, stir together the brown sugar, 1/2 cup flour, and cinnamon. Pour in the melted butter and mix until the mixture forms coarse crumbs. Set aside.

In a medium bowl, whisk together 2 cups flour, baking powder, baking soda, and salt until evenly combined.

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

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

Gradually add the dry ingredients to the butter mixture in three additions, alternating with the sour cream, starting and ending with the dry ingredients. Mix on low speed just until combined; do not overmix.

In a small bowl, toss the blueberries with 1 tablespoon of flour to help prevent them from sinking. Gently fold the berries into the batter using a rubber spatula.

Spread the batter evenly into the prepared pan. Sprinkle the crumb topping evenly over the top.

Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown. Let the cake cool in the pan on a wire rack for at least 20 minutes.

In a small bowl, whisk together the powdered sugar, milk (or cream), and vanilla until smooth. Drizzle over the cooled cake before slicing.