Preheat oven to 325 degrees. Spray a 9-by-12 1/2-inch baking pan with Pam, or Pam for baking. Set aside.
In a medium bowl, sift together 1 1/2 cups flour, granulated sugar, baking powder, and salt; set aside.
In a second bowl, whisk together egg, milk, canola oil, and vanilla. Using a rubber spatula, fold dry ingredients into egg mixture. Spread batter evenly into prepared pan, and set aside.
In a medium bowl, combine remaining 2 1/2 cups flour, brown sugar, and cinnamon. Pour melted butter over flour mixture, and toss with a rubber spatula until large crumbs form. Sprinkle crumbs over batter. Transfer pan to oven, and bake, rotating pan after 10 minutes. Continue baking until a cake tester comes out clean, about 10 minutes more. Total time should be 20-25 minutes, depending on your ovn.
Transfer baking pan to a wire rack to cool. Dust with confectioners' sugar. Using a serrated knife or bench scraper, cut into 3-inch squares. Store in an airtight container for up to 3 days.