• Preheat oven to 325°. Butter and flour a 9x13-inch baking pan or line with parchment and lightly grease with non-stick baking spray.

Make the Cake:

• In a medium bowl sift the flour, sugar, baking powder and salt together.
• In a separate large mixing bowl whisk the egg, milk, oil, and vanilla together until well combined.
• Add the dry ingredients to the wet and gently fold in with a spatula or wooden spoon.
• Scrape the batter into the greased pan and spread evenly from corner to corner - it will be a very thin layer.

Make the Topping:

• In a medium bowl whisk the flour, brown sugar and cinnamon together. Pour the melted butter over it and toss together with a spoon until well combined.
• Use your hand to sprinkle the crumbs evenly over the top of the batter breaking the mixture apart into big crumbs.
• Bake 10 minutes and rotate the pan. Bake 10 - 12 minutes longer or until a toothpick comes out clean from center.
• Cool in pan on a wire rack.
• Dust the top generously with confectioners' sugar and use a serrated knife to cut into squares or rectangles.
• Store tightly covered in a cool, dry place up to 4 days.