1-Preheat the oven to 350°F: Preheat the oven to 350°F. Spray an 8×8-inch (or 7×9-inch) baking dish (at least 2 inches deep) with nonstick spray and line with parchment paper, if desired.
2-In a large bowl, whisk together the: In a large bowl, whisk together the 1/2 cup melted butter or oil, 1/2 cup milk, 1/2 cup sugar, 1 egg, and 1 teaspoon vanilla.
3-Add 1 1/2 cups flour, 2 teaspoons: Add 1 1/2 cups flour, 2 teaspoons baking powder, and 1/2 teaspoon salt to the bowl and mix together with a wooden spoon. Gently fold in 1 1/2 cups blueberries until just combined. Spoon the batter into the baking dish.
4-In a small bowl, cut 1/4 cup: In a small bowl, cut 1/4 cup flour, 1 teaspoon cinnamon, 1/4 cup sugar, and 3 Tablespoons butter with a fork or your fingers until it forms clumps. Crumble evenly over the batter.
5-Cover dish loosely with foil and bake: Cover dish loosely with foil and bake 35 minutes. Remove foil and bake 5 to 10 more minutes, or until a toothpick inserted in the center comes out clean.
6-Remove from the oven and let cool: Remove from the oven and let cool at least 15 minutes. Slice into 9 squares and serve.