Preheat oven to 350 degrees and grease a 9X13-inch pan.
In a small bowl combine oats and boiling water. Let sit 15 minutes.
In a medium bowl, whisk together all-purpose flour, whole wheat flour, baking soda, baking powder, salt, cinnamon, and nutmeg. Set aside.
With electric mixer, beat butter until creamy. Add 1 cup light brown sugar and 3/4 cup granulated sugar. Beat until mixed well.
Beat in eggs one at a time.
Beat in vanilla.
With mixer on low speed, add flour mixture, alternating with oatmeal, mixing just until incorporated. Pour batter into prepared pan. Bake for 25 to 30 minutes or until a wooden toothpick inserted in the center comes out clean.
In a medium saucepan, melt 1/2 cup butter. Stir in 1 1/3 cup light brown sugar and half-and-half and stir until sugar is melted. Remove from heat and stir in coconut, pecans, and 1/2 teaspoon vanilla extract.
Pour icing on top of hot cake. Place under broiler until icing is bubbly and starting to brown, about 2 to 3 minutes. Cool in pan. Serve warm or at room temperature.