Mix together the flour, sugar, cocoa, baking soda and salt. Stir in the water, oil, vanilla and vinegar. Mix well and pour into 2 greased 12" x 17" (Costco) pans. Bake at 350° for 12-15 minutes.
Beat together the filling ingredients. Spread on one cake and then flip the second cake on top. Sprinkle with powdered sugar.