Preheat oven to 350 degrees and grease a glass 9x13 baking dish.
In a medium sized glass bowl melt the butter, then add the cake mix and water, mixing well.
Fold in the diced (or pureed) mangos and stir until the batter comes together. The batter should be quite thick but if you need to thin it with some water you can add some more a Tbs at a time to reach desired consistency.
Bake at 350 degrees for 35-40 minutes or until center of the cake springs back when touched.
While the cake is baking, make the pudding by adding milk, sifted flour, and sugar to a saucepan and heating over medium heat, whisking constantly, until mixture thickens.
Allow mixture to cool for about 20 minutes and then cover with plastic wrap, pushing the wrap down onto the surface of the pudding to keep it from forming a skin.
Allow both the pudding and the cake to cool completely (about 2 hours) and then poke holes in the cake with the handle of a wooden spoon.
Spoon the pudding over the cake and spread it out with a spatula. You want it to evenly coat the cake but not be too thick. You may not need all of the pudding.
Allow to sit for 5 minutes and then top with whipped topping. Cut into squares and serve!