Preheat oven to 350°.
To prepare cake, lightly spoon 1 1/2 cups flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (through salt) in a large bowl, stirring with a whisk. Make a well in center of mixture. Combine milk, melted butter, vanilla, and egg, stirring with a whisk; add to flour mixture, stirring just until moist. Fold in apple. Pour batter into an 8-inch square baking pan coated with cooking spray.
To prepare streusel, combine brown sugar, 2 tablespoons flour, and 1/2 teaspoon cinnamon; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle streusel evenly over batter. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack before serving. Serve warm.