"This is a moist and dense chocolate cake that is perfect for an after-school snack with a glass of milk. It's easy to make because there is no creaming of butter and sugar and no electric beater needed; everything is mixed by hand in one bowl. The frosting has a pudding-like consistency that fits the cake just right."
Preheat oven to 350. Grease and lightly flour two 8-inch round cake pans or one 9x13 pan. With non-stick pans, you can omit the flouring. Combine the flour, sugar, cocoa, baking soda and salt in a bowl. Add the milk or buttermilk, oil, egg and vanilla and mix with a wire wisk until smooth, about 1 minute. Spread batter in the pan(s) and bake for 20 minutes for the round pans or 35 minutes for the rectangular pan until the middle is set and a toothpick comes out clean. Do not overbake. Cool for 10 minutes in the pan before turning out.
Combine the chocolate, sugar and cornstarch in a pot. Stir in the boiling water and cook over medium low heat, stirring constantly with a wire wisk, until thick and smooth. This takes about 5 minutes and once the mixture starts to thicken it happens quickly. Remove from the heat and stir in the butter, vanilla and salt. Wisk well until smooth. This makes enough to fill and frost the two-layer cake (no side coverage) or the top of the larger single layer. Let the frosting cool a bit before putting it on the cake.