Crumb topping: 1. In a small bowl, mix together brown sugar, flour, and cinnamon. 2. Using a pastry blender or fork, cut in vanilla extract and unsalted butter. 3. Reserve 1/2 cup of the crumb topping and put aside. 4. Add chopped pecans to the remaining topping. Cake: 1. Preheat the oven to 375°F. Coat the 8x8-inch square pan with non-stick spray. 2. In a bowl, mix together flour, baking powder, and sugar. 3. Stir in the milk, egg, and oil. 4. Add in blueberries until just mixed. 5. Sprinkle 1/2 cup of the reserved crumb topping on the bottom of the baking pan. 5. With a spatula, carefully spread the cake batter in the pan. 6. Sprinkle the remaining reserved crumb topping over the rest of the cake batter. 6. Bake 25 minutes or until a toothpick inserted in the center comes out dry. 7. Serve warm from the oven. Or serve at room temperature.