1Preheat oven to 350 degrees. Grease and flour a metal 13x9-in baking pan.
2Prepare your streusel topping first. In a medium bowl, combine brown sugar, flour, and butter. With your fingers, work in the butter until the mixture is crumbly. Refrigerate while preparing the cake.
3In a bowl, combine flour, baking powder, baking soda, and salt - set aside. In your mixer, combine butter and sugar. Blend until creamy at medium-high speed, about 2 minutes. Reduce speed, add eggs 1 at a time, blending well after each addition. Add vanilla.
4At low speed, beat in flour mixture alternately with the buttermilk, beginning and ending with the flour mixture. Beat just until batter is smooth, occasionally scraping bowl with a rubber spatula. Fold in all but 1/2 cup of the blueberries.
5Spoon batter into your prepared pan; spread evenly. Sprinkle batter with reserved blueberries. With hands, sprinkle streusel topping evenly over batter.
6Bake 30-40 minutes, depending on your oven. Test the cake with a toothpick to see when it comes out clean. Cool in pan on a wire rack for 1 hour to serve warm, or cool completely to serve later.