Preheat oven to 350 degrees F (175 degrees C). Grease a 12x18-inch jelly roll pan.
Step 2
Sift flour, white sugar, and salt together in a large bowl.
Step 3
Combine 1 1/2 cups margarine, water, and 4 1/2 tablespoons cocoa, together in a saucepan; bring to a boil. Pour margarine mixture over flour mixture; stir to combine.
Step 4
Beat buttermilk, eggs, baking soda, and vanilla extract together in a bowl; add to margarine mixture and stir until batter is combined. Pour batter into prepared pan.
Step 5
Bake in the preheated oven until a toothpick inserted into the center of the brownies comes out clean, about 20 minutes. Cool completely.
Step 6
Heat 1/2 cup margarine, milk, and 3 tablespoons cocoa powder together in a small saucepan over low heat until margarine melts and mixture is combined; remove from heat. Beat confectioners' sugar into margarine mixture until frosting is smooth and creamy. Spread frosting over cooled brownies and cut into squares.
