Blueberry Buckle

1. FOR THE STREUSEL: Using stand mixer fitted with paddle, combine flour, brown sugar, granulated sugar, cinnamon, and salt on low speed until well combined and no large brown sugar lumps remain, about 45 seconds. Add butter; beat on low speed until mixture resembles wet sand and no large butter pieces remain, about 2-1⁄2 minutes. Transfer to bowl; set aside.
2. FOR THE CAKE: Adjust oven rack to lower-middle position and heat oven to 350 degrees. Grease 9-inch round cake pan, line bottom with parchment paper, grease parchment, then flour pan.
3. Whisk flour and baking powder together in bowl; set aside. Using stand mixer fitted with paddle, beat butter, sugar, salt, and zest on medium-high speed until light and fluffy, about 3 minutes, scraping down bowl as necessary. Beat in vanilla until combined, about 30 seconds. With mixer on medium speed, add eggs 1 at a time; beat until partially incorporated, scrape down bowl, and continue to beat until fully incorporated (mixture will appear broken). With mixer on low speed, gradually add flour mixture; beat until flour is almost fully incorporated, about 20 seconds. Stir batter with rubber spatula, scraping bowl, until no flour pockets remain and batter is homogeneous; batter will be very heavy and thick. Gently fold in blueberries until evenly distributed.
4. Transfer batter to prepared pan. Spread batter evenly to pan edges and smooth surface. Squeeze portion of streusel in hand to form large cohesive clump; break up clump with fingers and sprinkle streusel evenly over batter. Repeat with remaining streusel. Bake until cake is deep golden brown and toothpick inserted in center comes out clean, about 55 minutes, rotating pan halfway through baking. Transfer pan to wire rack and let cool, 15 to 20 minutes (cake will fall slightly as it cools).
5. Run paring knife around edges to loosen. Invert cake, then peel off and discard parchment. Invert cake onto serving platter. Let cool at least 1 hour. Cut into wedges and serve warm or at room temperature. (Buckle can be stored at room temperature, wrapped in plastic wrap, for up to 2 days.)

The batter will be extremely thick and heavy, and some effort will be required to spread it into the prepared pan. Be sure to use a cake pan with at least 2-inch-high sides. This buckle is best made with fresh blueberries, not frozen ones, which are too moist. Serve with vanilla ice cream.

INDIVIDUAL BLUEBERRY BUCKLES
Line 12-cup muffin tin with paper or foil liners. In step 4, transfer batter to prepared tin; spread batter evenly to cup edges and smooth surface. (Batter will reach top of liners.) Reduce baking time to 35 minutes. Let buckles cool in pan on wire rack for 10 minutes. Remove from tin and let cool, at least 30 minutes. Serve warm or at room temperature.