Directions: preheat oven to 350 degrees. Mix together the cake mix, sugar and flour in a large mixing bowl. add in wet ingredients: water, sour cream, oil, mango and eggs. beat on low until mixed then bet on medium/high for 2 minutes. line cupcake pan with paper cups and fill 2/3 way full with batter. Turn oven down to 325 degrees and bake for about 25-35 minutes or until a toothpick comes out clean.
2For the Filling
in a medium saucepan, combine all ingredients and wisk over med/high heat until mixture thickens, take off heat cover and chill.
3For the Frosting
combine butter, shortening, vanilla, lime juice and mango in a large mixing bowl and beat on high for 5 minutes. Turn mixer to low, Add the powdered sugar little by little until mixture becomes thick and creamy and there are no lumps. add more or less powdered sugar as needed to get desired consistency.
4Putting the Cupcake together
Once the cupcakes are cooled and the custard is chilled, place the custard in a piping bag with a large tip (I use wilton #12 tip). Poke the tip 2/3 of the way into the cupcake and gently squeeze filling into the center of the cupcake being careful not to bust the cupcake, the filling should push the tip up and out the top of the cupcake as it fills the center. Do this for every cupcake. Place rosting in piping bag with whatever tip you would like and frost away. Garnish with key lime zest (or sugar sprinkles)