Streusel Topping

• ½ cup whole-grain pastry flour
• ½ cup spelt flour
• 3 tablespoons sugar
• 1 tablespoon dark brown sugar
• ½ teaspoon baking powder
• ½ teaspoon ground cinnamon
• ¼ teaspoon kosher salt
• 3 tablespoons cold unsalted butter
• 1 egg

Dry Mix

• 1¼ cups spelt flour
• 1 cup whole-grain pastry flour
• ½ cup dark brown sugar
• ½ cup sugar
• 1 tablespoon baking powder
• 1 teaspoon cinnamon
• 1 teaspoon kosher salt
• 4 ounces (1 stick) unsalted butter, softened

Wet Mix

• ¾ cup whole milk
• ⅓ cup plain yogurt
• 4 egg yolks
• 2 teaspoons vanilla extract
• 2 cups blueberries or huckleberries

1. Preheat the oven to 350°F. Butter a 2½-quart baking dish.
2. For the streusel topping, sift the flours, sugars, baking powder, cinnamon, and salt into a large bowl, pouring back into the bowl any bits of grain or other ingredients that may remain in the sifter. Cut the 3 tablespoons of cold butter into ¼-inch pieces and add them to the dry mixture. Rub the butter between your fingers, breaking it into smaller bits. Continue rubbing until the mixture feels coarse, like cornmeal. The faster you do this, the better.
3. Whisk the egg thoroughly. Use a spatula to scrape the egg over the dry ingredients, then use your hands to mix the egg in. Squeeze handfuls of the dough together. The streusel topping should be in hazelnut-size clumps mixed with finer crumbs.
4. For the batter, place a sifter over the bowl of a standing mixer and sift the dry ingredients, pouring back into the bowl any bits of grain or other ingredients that may remain in the sifter. Add the softened butter to the bowl and attach the paddle. On medium speed, blend the butter into the flour mixture until combined.
5. In a large bowl, whisk together the milk, yogurt, egg yolks, and vanilla until thoroughly combined. Pour the milk mixture into the mixing bowl and paddle on low speed until the batter is smooth.
6. Scrape half the batter into the buttered dish. Sprinkle half of the berries over the surface. Scrape the remaining batter over the berries and gently spread it evenly in the pan. Sprinkle the last half of the blueberries over the batter and top with the streusel mixture.
7. Bake for 55 to 65 minutes, rotating the pan halfway through. The cake is done when it is golden brown and springs back when lightly touched, or when a skewer inserted in the center comes out clean. Allow the buckle to cool in the pan. It can be eaten warm, at room temperature, or cooled, wrapped tightly in plastic, and kept for up to 2 days.