Pour boiling water into a large glass measuring cup. Stir in the oats. Place the chocolate morsels over the top of the oatmeal -- do not stir!, Let stand for 20 minutes until the oats are soft and the chocolate is melted.
In a large bowl, cream the butter and gradually beat in the sugars. Beat in eggs one at a time. Add the oats and melted chocolate, mixing well. Add the flour, salt and baking soda. Mix thoroughly.
Transfer the batter to a greased and floured baking pan (13 by 9 by 2-inches). Bake in a 350 degree oven for 40 minutes, or until the cake starts to leave the sides of the pan and springs back at the touch of a finger. Cool 10 minutes in pan, shake loose, and turn out on rack to cool completely. Frost and decorate if desired.
In a custard cup, dissolve the instant coffee in the milk or cream. In a bowl, cream butter until light, add vanilla, salt and gradually blend in sifted confectioners' sugar with the coffee-milk mixture. Use enough sugar for a good spreading consistency.
Yield: frosting for the top of a 13 by 9-inch cake