Greg’s Blueberry Crumble Cake
6 tablespoons (3/4 stick) unsalted butter, preferably cultured, melted plus more for the pan
1/3 cup buttermilk, at room temperature
1 large egg, at room temperature
1 large egg yolk, at room temperature
1/3 vanilla bean, split lengthwise and seeds scraped, pod reserved for another use
1 ½ cups all-purpose flour
¾ cup granulated sugar
1 ½ teaspoons baking powder
¾ teaspoon kosher salt
1 pint blueberries
Spiced Crumble Topping
½ cup all-purpose flour
3 tablespoons granulated sugar
1 teaspoon ground cinnamon
¼ teaspoon ground allspice
3 tablespoons unsalted butter, preferably cultured, cup up and chilled
1 tablespoon raw sugar
1 teaspoon Maldon salt or other flaky sea salt
To make the cake, preheat the oven to 375F. Butter a 9x5-inch nonstick loaf pan.
In a medium bowl, whisk together the buttermilk, egg, yolk, and vanilla seeds until well blended. Continue whisking while drizzling in the butter.
In a large bowl, whisk together the flour, sugar, baking powder and salt. Add the buttermilk mixture and whisk until just blended. Use a rubber spatula to fold in the blueberries. Transfer to the prepared pan, smoothing the top.
To make the topping, whisk together the flour, granulated sugar, cinnamon, and allspice in a medium bowl. Rub in the butter using your fingers until the mixture is crumbly with a few pea-sized pieces remaining. Sprinkle evenly over the cake batter. Sprinkle the raw sugar and Maldon salt evenly over the topping.
Bake until deep golden brown and a tester inserted in the center of the cake comes out clean, about 1 hour. Let cool completely in the pan on a wire rack. Unmold and cut into slices to serve.