Preheat oven to 375 degrees Fahrenheit. Grease a 9x9-inch baking pan. (I like to use Baker's Joy.) Set aside.

For the streusel: Place flour, sugar, cinnamon and butter in a bowl. Cut ingredients together with a pastry blender or fork. Or simply rub the mixture together with your fingers until the butter is incorporated. (There should be some fine crumbs, but it should be able to clump together if you squeeze it in your hands.) Refrigerate streusel until ready to assemble cake.

For the cake: In a separate bowl, sift together the flour, baking powder, and salt. Set aside.

In a stand mixer with the paddle attachment, cream the butter and sugar together until light and fluffy.

Incorporate the eggs one at a time, then mix in the vanilla. Scrape bowl.

Add HALF the dry ingredients. Mix until just incorporated.

Add all of the buttermilk. Mix until just incorporated.

Add the remaining dry ingredients. Mix until just incorporated. Do not over-mix.

Gently fold in the blueberries. Spread cake batter evenly into the prepared pan. Sprinkle the top with the prepared streusel. (I like to squeeze some of the streusel in my hands to have some big clumps, and not all just sandy crumbs.)

Bake at 375˚F for 45 to 60 minutes or until the cake springs back (doesn’t jiggle) when touched in the center. Do not over-bake. (NOTE: The streusel will scrape the cake batter off a toothpick even if cake is underbaked. Carefully use your fingers to make sure the cake doesn't jiggle in the center.)

Allow cake to cool completely before cutting and serving. Store cake covered tightly in pan with foil or plastic wrap. Alternately transfer cake to an airtight container and store at room temperature up to 5 days.