Preheat oven to 350 degrees F. Spray a 9 x 13 pan generously with cooking spray.

In a large bowl, whisk together the flour, sugar, baking powder and salt. Drop in the butter, separating as you do. Toss the butter in the flour mixture, then working with a pastry cutter or your fingers, cut the butter into the flour until the mixture looks like sand.

Add the egg to the milk and stir. Add to the dry ingredients. Stir until just combined and moistened. Transfer to prepared baking dish and smooth.

In a small bowl, mix together the brown sugar and cinnamon. Starting at the edges, sprinkle this mixture over the batter in the pan. Shake the pan gently to distribute the topping evenly. Scatter the small cubes of butter over the topping. Press each cube into the topping so they are barely visible.

Bake for 23  to 25 minutes or until the top, on a drier spot, springs back up when gently pressed and the edges are slightly brown. The toothpick test is unreliable for this cake.

Notes: May also be baked in 8 or 9″ round pans; check for doneness a few minutes early and watch carefully; timing has not been tested.