Preheat oven to 350 degrees F.

Batter: Mix sugar and oil in a mixing bowl. Add eggs one at a time, mixing well after each egg is added. Add vanilla. Add dry ingredients: cocoa powder, salt, flour, and stir until combined.

Bake: Pour batter into a greased 9×13'' baking pan and spread into an even layer. Bake at 350 degrees F for 30 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before frosting.

Mint Frosting: Combine all frosting ingredients and beat until light and fluffy. Add more milk if you want it thinner. Spread the frosting evenly over the cooled brownies. Refrigerate while you make the chocolate frosting.

Chocolate Frosting: Combine butter and cocoa powder. Add powdered sugar, milk, and vanilla extract. Mix for several minutes until smooth and fluffy.