Place rack in center of oven and heat to 325 degrees.
Lightly brush a 9 by 12 1/2-inch baking pan with canola oil, dust with flour and tap to remove excess. 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 until just combined.
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 and bake, rotating pan after 10 minutes. Continue baking until a cake tester comes out clean, about 10 minutes more.
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 up to 3 days.
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