Source  Brown Eyed Baker

Recipe Added:  7/20/2013

Topping  With electric mixer, combine flour, sugars, cinnamon, and salt on low speed until combined, about 45 seconds. Add butter and mix on low until mixture resembles wet sand and no large butter pieces remain, about 2 minutes. Transfer to a small bowl and set aside.

Cake  Preheat oven to 350F. Grease a round 9-inch cake pan, line bottom with parchment, grease parchment, flour inside of pan. In a small bowl, whisk flour and baking powder; set aside. Using an electric mixer, cream butter, sugar, salt on medium-high until light and fluffy, 3 minutes. Add vanilla and lemon and beat until combined, about 30 seconds.

Reduce speed to medium, add eggs one at a time; beat 5-10 seconds, continue until fully incorporated. Reduce speed to low, gradually add flour mixture and beat until almost fully incorporated, 20 seconds. Use a rubber spatula to finish mixing until no flour pockets remain (batter will be very thick). Using rubber spatula, gently fold blueberries into batter until evenly distributed.

Transfer batter to prepared pan; use a spatula to gently spread batter. Sprinkle streusel over batter. Bake until top is golden brown and toothpick inserted into center comes out clean, about 55 minutes. Place on wire rack 20 minutes Run a thin knife around the sides of cake. Invert, then peel off parchment paper. Turn cake right-side-up onto a cooling rack. Cool 1 hour. Serve at room temperature.

If using frozen blueberries, measure frozen berries into bowl, mix in a few spoonfuls of flour to absorb liquid as they soften. Keep frozen until ready to add to batter.

1. http://www.browneyedbaker.com/2013/07/15/blueberry-buckle-recipe/