Beat the butter and sugar until creamy. Add eggs, vanilla, salt, and cocoa. Mix baking soda in buttermilk until it starts to fizz. Pour buttermilk into batter. Stop mixer and add flour. Mix until smooth, about 3 minutes. Add boiling water and mix. Pour batter into long cake pan 13 by 9 by 2. Bake at 375 degrees for 35 minutes. Test if it's done using a toothpick. If toothpick comes out clean, it's done. If it doesn't come out clean, leave in a few more minutes and retest.
1/2 box powdered sugar (1 lb. size); 1 stick butter; 2 level TBSP Hershey's Cocoa; 2 tsp vanilla; 1.5 TBSP milk. Feed in mixer. The longer you beat it the creamier it gets. Cool cake first before frosting. For vanilla icing, use same recipe but omit cocoa.