Preheat oven to 375°F. Spray an 8x8-inch baking dish with baking spray.
Add the blueberries with 1/3 cup of sugar, 1 tablespoon of flour, and cinnamon to the baking dish and toss to evenly coat the berries.
3 cups blueberries, 1/3 cup granulated sugar, 1 Tablespoon all-purpose flour, 1/4 teaspoon ground cinnamon
In a large bowl, beat the butter, brown sugar, and granulated sugar using a mixer until creamy, about 2 minutes.
6 Tablespoons salted butter, 1/2 cup granulated sugar, 1/4 cup brown sugar
Add the egg and vanilla and beat well.
1 large egg, 1 teaspoon vanilla extract
Add the flour, baking powder, and salt, mixing until combined. Add boiling water and mix well. Dollop spoonfuls of the thick batter of the blueberry mixture. It's okay for some of the berries to show through. Sprinkle the top of the batter with about 1 tablespoon of granulated sugar.
1 cup all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon salt, 1/4 cup boiling water
Bake 40 to 45 minutes or until top is light golden brown and filling is bubbling.