Combine oil, cocoa and sugar in mixing bowl and beat on low speed until well-mixed. Add eggs; beat on medium speed until well blended. Add salt, buttermilk, flour and vanilla. Add to mixture on low speed until combined; then beat on high speed for 2 minutes. Add soda to hot water; dissolve well. Add gradually to cake batter and mix just until combined. (Batter will be very thin before baking.) Bake in greased and floured 13x9 inch baking pan or 2 layer pans at 190ºC or 40 minutes. Test with a toothpick in center of cake for doneness. Frost with Chocolate Fudge Frosting, if desired.