Heat oven to 375 degrees F. Spray an 8x8-inch glass baking dish with cooking spray.
In a medium bowl, sift together 2 cups cake flour, baking powder and salt. In a large bowl, using a mixer on medium speed, beat together butter, sugar and vanilla until creamy. Beat in egg until well combined. Reduce mixer speed to low and beat in flour mixture, alternating with milk, until just combined. Gently fold in blueberries. Spread batter into the bottom of prepared pan.
To make the topping, beat together remaining 1/4 cup butter, 1/3 cup cake flour, brown sugar and cinnamon until well combined and crumbly. Sprinkle crumble on top of cake batter. Bake 35-40 minutes or until well risen and golden brown. A toothpick inserted into cake should come out with moist crumbs attached.