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.