Heat oven to 350 degrees F. Mix flour, brown sugar, cocoa, baking soda, and salt with a fork in an ungreased square pan; 8x8x2 inches.
Mix in remaining ingredients except chocolate chips.
Sprinkle chocolate chips over batter.
Bake until wooden pick inserted in center comes out clean, about 35-40minutes.
Sprinkle with powdered sugar, if desired.