Preheat oven to 375 degrees.
In mixing bowl, stir together flour, sugar, cocoa, baking soda and salt.
Add lemon juice to milk and let stand briefly to sour milk.
Add milk to dry ingredients with oil and vanilla.
Beat until batter is smooth.
Pour into a greased and floured 9 by 9 by 2 inch baking pan and place in oven for 30 minutes.
Cool in pan on a rack.
Sprinkle with confectioners sugar (OPTIONAL)
Cut into squares to serve.