Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, firmly packed light brown sugar, baking powder, and salt in a large bowl.
Place butter in a small skillet over medium heat. Cook 6 minutes or until lightly browned, stirring occasionally. Pour into a small bowl, and cool 10 minutes.
Combine butter and egg substitute, stirring with a whisk. Pour butter mixture over flour mixture; stir just until moistened. Spoon batter into a 13 x 9-inch baking pan coated with cooking spray; smooth top with spatula. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Cut into 24 squares.
Store unrefrigerated in an airtight container up to three days, or freeze for three months.