TO MAKE BROWNIES: Preheat oven to 350. Grease and flour a 9 x 13 pan. Mix together flour, sugar, baking soda and salt. Set aside.
Melt margarine, cocoa and water. Bring to a hard, rapid boil. Pour over dry ingredients and mix. Add buttermilk, eggs, vanilla and pecans. Pour into a greased and floured 9 x 13 pan. Bake at 350 for 30 minutes or tuntil a toothpick inserted in the center comes out clean.
TO MAKE FROSTING: Bring to a boil butter, cocoa and buttermilk. Add and beat confectioners sugar, vanilla extract and pecans. Spread on top of the warm brownies in pan. Cool in refrigerator.