FOR THE TOPPING
1/2 cup sugar
1/3 cup cake flour
1/2 teaspoon freshly grated nutmeg
4 tablespoons unsalted butter, cubed and chilled
FOR THE CAKE
2 cups cake flour
1 teaspoon baking powder
1/2 teaspoon kosher salt
1/2 teaspoon ground ginger
4 tablespoons unsalted butter, at room temperature
3/4 cup sugar
1 large egg
1/2 cup whole milk
3 cups fresh blueberries
Combine the sugar, flour, and nutmeg in a small bowl then work in the butter, using a fork or pastry cutter to combine. Work until the mixture has a crumb-like texture. Set aside.
Heat the oven to 375ºF. Spray a 9-inch square glass baking dish with nonstick cooking spray and set aside.
Whisk the flour, baking powder, salt and ginger together in a medium bowl and set aside.
In a stand mixer fitted with a paddle attachment, beat together the butter and sugar on medium speed until light and fluffy, approximately 1 minute. Reduce the speed a bit and thoroughly incorporate the egg. Reduce speed to low and add one-third of the flour mixture. Once incorporated, add one-third of the milk then repeat alternating until all ingredients are combined.
Gently stir in the blueberries and pour the mixture into the prepared baking dish and sprinkle on the topping mixture.
Bake on the middle rack of the oven until golden, about 45 minutes. Cool for 30 minutes before serving.