Whipped cream and additional blueberries* for garnish
Begin by preheating your oven to 350 degrees F. Spray a 9x13-inch pan with baking spray. Set aside.
In a large bowl, whisk together the flour, sugar, and cinnamon. Then, with a pastry blender (or fork) cut in the butter until pea-sized. Measure out 3/4 cup of the mixture and set aside. Stir baking powder and salt into the remaining mixture. Then, with an electric mixer set on low speed, mix in the egg yolks and buttermilk for about 3 minutes.
In a large glass or metal bowl, whip the eggs whites until stiff peaks begin to form. Then, gently fold the egg whites into the batter. Spread the batter evenly in the prepared pan. Sprinkle blueberries over the batter, followed by the reserved crumb mixture.
Bake for 45-50 minutes or until the cake springs back when lightly touched in the center.
Serve with a dollop of whipped cream and additional blueberries.
*I used frozen wild blueberries, but fresh will work as well.