For the Cake:
2 cups all-purpose flour
2 teaspoons baking powder
1/2 teaspoon salt
1/4 cup unsalted butter, softened
3/4 cup granulated sugar
1 large egg
1 teaspoon vanilla extract
1/2 cup milk
2 cups fresh blueberries (or frozen, if fresh are not available)
For the Streusel Topping:
1/2 cup granulated sugar
1/3 cup all-purpose flour
1/2 teaspoon ground cinnamon
1/4 cup unsalted butter, cold and cubed