Preheat oven to 375ºF. Butter and flour a 9 x 9-inch baking pan. Set aside. In a double boiler, melt chocolate and butter, stirring until smooth. Stir in vanilla and let cool slightly. In a large mixing bowl, beat eggs and sugar on medium high speed until thick and creamy, and sugar is dissolved, about 10 minutes. Gently stir in melted chocolate. Stir salt into flour and fold gently into chocolate mixture. Do not over mix. Pour batter into prepared pan and spread to smooth. Bake 20 minutes, then cover with foil. Bake additional 8 to 12 minutes or until a toothpick inserted 1 inch from the center comes out clean.
While the brownies are baking, prepare the frosting. Melt butter and stir in cocoa powder. Mix until well blended. Add warm water and vanilla and whisk in confectioner's sugar, a little at a time, until frosting is smooth and spreadable, but not too thick.
Remove brownies from oven and let cool slightly in pan. Spread frosting over warm brownies for a hot fudge sauce effect or spread on cooled brownies and refrigerate until firm for brownies that will be served later or packed for travel.