To make the cake, mix together sugars, flours and salt in large mixing bowl. Place butter, coconut oil, cocoa and water in a medium saucepan. Bring to a rapid boil, remove from heat and let cool slightly. Pour over flour mixture. Mix until well blended, then mix in eggs, buttermilk, soda and vanilla. (Makes a thin batter.)
Pour batter into 13 x 9 x 2-inch baking pan and bake 30 to 35 minutes. Remove and let cool.
To make the icing, melt butter in a small saucepan over low heat. Remove from heat and whisk in cocoa, salt, and cream. Whisk in powdered sugar until smooth and pour over cooled cake.