Pour the boiling water over the oats and let them stand for 20 minutes while heating the oven to 350°F and greasing a 9-by-13-inch pan.
Beat the softened butter, granulated sugar and 1/2 cup brown sugar until combined, then beat in the eggs and soaked oats.
Whisk the flour, baking soda, salt and cinnamon and fold them into the wet mixture.
Spread the batter in the pan and bake for 32 to 35 minutes until the center springs back.
Mix the melted butter, milk, remaining 1/2 cup brown sugar, coconut and pecans and spread it over the hot cake.
Broil 5 inches from the heat for 1 to 3 minutes, watching constantly, until the topping bubbles and browns at the peaks.