In a 9 x 13 inch cake pan, stir together, flour, brown sugar, cocoa, baking soda and salt.
Make a well in the center of the dry ingredients. Add milk, oil, lemon juice and vanilla. Beat with a fork until smooth, about 2 minutes. Use a rubber spatula to bring any dry ingredients clinging to sides, corners, or bottom of the pan into the batter. Level the batter in the pan with the spatula.
Bake at 350 F for 45 to 50 minutes or until it tests done. Cool completely. Cut into squares and dust with icing sugar if desired. Makes 12 to 18 servings depending on how you cut them.