Step 1Put the water in a saucepan and bring to a boil. Place the oatmeal in a bowl. When the water comes to a boil, pour the water over the oatmeal and stir. Cover the bowl and let the oatmeal stand for about 20 minutes, then uncover and stir. The oats should have cooled down so they're not hot when you add them to the batter.
Step 2Heat the oven to 350 degrees. Grease a 13x9-inch cake pan or muffin tins.
Step 3You can either mix this batter using an electric mixer, which is easier and quicker, or beat the batter by hand using a large mixing spoon. Place the butter in a mixing bowl and gradually add the granulated and brown sugars, beating with each addition until the mixture is blended and creamy. Add the vanilla and eggs and mix well. Add the oatmeal and mix until the oats are well blended in the batter.
Step 4In a separate bowl, combine the flour, baking soda, salt, cinnamon and nutmeg until well blended, then add them to the butter mixture. Mix thoroughly so the batter is all one color.
Step 5Spoon the batter into the cake pan or muffin tins. (The muffin cups should be half full of batter.)
Step 6Bake the cake until it's lightly browned and springy to the touch and a toothpick inserted in the middle comes out clean, 45 to 50 minutes for the cake, 20 minutes for the cupcakes. Test the cake at 40 minutes. Test the cupcakes after 15 minutes. Cool the cake on a wire rack.
Step 1Melt the butter and pour it into a mixing bowl. Add the brown sugar and half-and-half and mix well. Add the nuts and coconut and mix well. The frosting will be very thick.
Step 2Spread the frosting evenly over the cooled cake or cupcakes.
Step 3If you want to have the frosting finished with a sheen, put the frosted cake or cupcakes under the oven broiler about 6 inches from the broiler element and heat until the frosting bubbles a little, about 3 to 4 minutes. Keep an eye on it so it doesn't burn.