Preheat the oven to 350 degrees F. Grease and flour a 13 x 9-inch cake pan.
To prepare the cake, pour 1¼ cups boiling water over the oats in a bowl. Let stand for 20 minutes. Cream the butter, sugar, and brown sugar with a mixer. Add the oats; mix well. Add the eggs and vanilla; mix well.
Sift together the flour, baking soda, salt, and cinnamon. Add to the oats mixture; mix well. Pour into the prepared pan. Bake for 30 to 35 minutes, until browned. Preheat the broiler.
To prepare the topping, combine the butter, cream, brown sugar, and coconut in a medium bowl; mix well. Spread evenly over the warm cake. Place the cake under the broiler 2 inches from the heat source. Broil until lightly browned, about 1 to 1½ minutes. Cool on a wire rack.