Preheat your oven to 350° F
Generously butter a 2 ½ quart baking dish (or the equivalent). Here, I used a 12" cast iron pan)
In a large bowl, sift together the flours, sugars, cinnamon, baking powder, and salt
Add the butter cubes and use your fingers to rub them into the mix until it resembles the texture of cornmeal. Using a spatula, incorporate the whisked egg into the dough.
Using your hands, clump pieces of the dough together until small clumps of topping have formed. (Your clumps should be roughly the size of hazelnuts, with finer crumbs also in the mix)
Set the topping aside
Sift the crumble cake's dry ingredients into the bowl of a stand mixer fitted with a paddle attachment.
Add the softened butter and blend over medium speed until it is just incorporated
In a medium-sized bowl, combine the milk, yogurt, egg yolks, vanilla extract (or seeds scraped from the vanilla bean). Add the milk/yogurt mix to the mixing bowl and paddle on low until the batter is smooth.
Spread half of the batter in your baking dish. Sprinkle with 1 ½ - 2 cups of the berries
Add the second layer of batter and top with the rest of the blueberries. Sprinkle the streusel over the top
Bake the crumble cake for ~ 55- 65 minutes, rotating halfway through, until it’s lightly golden and a toothpick inserted into the center comes out clean.
Let the cake cool in the baking dish before serving