Heat oven to 325 degrees (alternately dependent on the pan you use, heat according to cake mix instructions)
Lightly grease a 9x13" baking pan
Prepare cake mix batter according to package instructions. You can really use any flavor of cake mix that you want but the coconut pairs great with the cinnamon.
Pour HALF of the batter into pan
Mix together cinnamon and brown sugar until there are no lumps (I used a fork to mix well)
Sprinkle cinnamon mixture over entire area of the cake so batter layer is completely covered
Pour the remaining batter over the cinnamon mixture
Bake according to package instructions, I did 29-30 minutes at 325 degrees, until toothpick inserted in center comes out clean
While cake is cooking, whisk together the powdered sugar and milk in a bowl till it is smooth and there are no lumps
When cake is done, immediately poke holes in the top of the cake with a fork. The more holes, the better, as the glaze will absorb into the cake more
Pour powdered sugar mixture over the top of the cake, evenly filling all of the holes
Cover and refrigerate at least 2 hours
Frost with Cool Whip, cover and return to refrigerator.
Toast coconut in a small, heavy non-stick pan over medium heat. You must stir coconut constantly to avoid burning. It should take about 2 minutes once the pan is heated. You can add the coconut right before serving or after you frost with Cool Whip just keep in mind it will soften a bit when refrigerated.
Cake is best if refrigerated 24 hours before serving. Serve with hot caramel topping (and ice cream, if you wish!)