In a 2-quart saucepan over medium heat, combine the milk and coconut milk and bring to a simmer. In a large mixing bowl, combine the egg yolks with 1/2 cup of the sugar and the flour and whisk until well-blended and pale yellow in color. Add about 3 of the heated milk mixture to the bowl with the egg mixture and stir to combine. Pour the contents of the bowl into the saucepan, and cook, stirring, until the mixture thickens and coats the back of a spoon, 3 to 4 minutes. Do not allow the mixture to boil. Once thickened, remove the saucepan from the heat and add the white chocolate, whisking until the chocolate has melted and is completely combined. Add the coconut to the pan and stir to blend. Transfer hot mixture to a large, heatproof mixing bowl and place a piece of plastic wrap over the pastry cream to prevent a skin from forming while it cools. Cool to room temperature before proceeding.
Using a pastry brush, coat the inside of each of 8 (4-ounce) ramekins with some of the melted butter. Using 4 tablespoons of the sugar, lightly coat the inside of each of the buttered ramekins with some of the sugar. Set aside until ready to use.
Preheat the oven to 425 degrees F.
Remove the plastic wrap from the pastry cream, and, using a hand held mixer with a whip attachment, beat the pastry cream until smooth. Add the rum to the pastry cream and whisk to incorporate. In a separate medium mixing bowl, add the egg whites. Using cleaned whip attachments for the mixer, beat the egg whites until they begin to turn white and foamy, 2 to 3 minutes. Gradually add the remaining 2 tablespoons of sugar to the bowl, and continue to beat the egg whites until soft peaks develop. Using a large plastic spatula, fold 1/3 of the egg whites into the pastry cream. Gently fold the remainder of the egg whites into the pastry cream base in 2 additions just until incorporated. Divide the souffle batter evenly among the 8 ramekins and place them on a sheet pan. Bake the souffles until they have risen at least 1-inch from the rim of the ramekins and the centers are set, about 12 to 14 minutes.
Serve the souffles as soon as they come from the oven, dusted with powdered sugar and with the Mango-Rum Sauce spooned over the top.
Yield: 8 servings