Line a 9 by 13-inch baking pan with foil so that it covers the bottom and overhangs 2 sides by at least 1-inch. Tuck the overlap around the outside of the pan. Coat with nonstick vegetable spray.
Set a rack in the middle of the oven and preheat to 350 degrees F.
In a bowl, combine 1 cup of the flour, 1/2 cup of the brown sugar, and the pecans. Work in the butter, rubbing and pinching it between your fingertips, until the mixture looks mealy and lumpy. Lightly pat the streusel onto the bottom of the pan.
In a mixing bowl, beat together the eggs and the remaining 1/2 cup brown sugar until doubled in volume. Beat in the vanilla extract.
In another bowl, sift together the remaining 1/2 cup flour, baking powder, and salt, then beat into the egg mixture. Spread on top of the streusel.
Sprinkle on the coconut and the sweetened condensed milk. Bake for 25 to 30 minutes, until the first tiny bubbles come up. Set on a rack to cool. Run a knife around the 2 unfoiled pan sides to loosen, then lift the whole cake out of the pan. Cut into 2-inch bars.
Recipe courtesy Wayne Harley Brachman, Retro Desserts, Harper Collins Publishers, 2000