Combine the first five ingredients in a large bowl; set aside. In a heavy saucepan, bring the butter, water and cocoa to a boil; add to dry ingredients and mix well. Beat in the eggs, buttermilk and vanilla. Pour into a greased 15x10x1-in. baking pan.
Bake at 350° for 25-30 minutes or until a toothpick inserted in the center comes out clean.
For frosting, in a small saucepan, combine flour and milk until smooth. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Cool.
In a small bowl, cream the remaining ingredients for 4 minutes. Add cooled milk mixture and beat until fluffy. Frost cake.