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1
Preheat oven to 350u00b0F. Grease a 13 by 9 inch metal baking pan; dust pan with flour or use baking spray that has flour in it such as Baker's Joy.
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2
Prepare streusel topping: In a medium bowl, combine brown sugar, 1/2 cup flour, and cold butter. With your fingertips, work in butter until mixture is crumbly. Refrigerate streusel while preparing cake.
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3
Prepare the coffee cake: On waxed paper or flexible mat, combine the remaining 2 1/2 cups flour, baking powder, baking soda, and salt. In a large bowl with the mixer at low speed, beat the granulated sugar with the softened butter until blended. Increase speed to medium-high and beat until creamy, about 2 minutes, frequently scraping the sides of the bowl with a rubber spatula. Reduce speed to low; add eggs, 1 at a time, beating well after each addition. Beat in vanilla.
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4
At low speed, beat in flour mixture alternately with buttermilk, beginning and ending with the flour mixture. Beat just until batter is smooth, occasionally scraping the bowl with a rubber spatula. With a spatula, fold in all but 1/2 cup blueberries.
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5
Spoon batter into prepared pan; spread evenly. Sprinkle batter with reserved blueberries. With fingers, press streusel into clumps and sprinkle evenly over the batter. Bake coffee cake 35 to 40 minutes or until a toothpick inserted in the center comes out clean. Cool cake in the pan on a wire rack 1 hour to serve warm, or cool completely to serve later.
