1. Preheat oven to 350°F.
2. In large bowl, mix cake mix, butter, oil and eggs on medium-low speed for one minute, then on high speed for on additional minute.
3. Pour into lined or greased cupcake pans (I used a mini cupcake pan for this recipe.)
4. Bake until golden brown or cupcakes pass the toothpick test.
5. While cupcakes are baking, prepare the filling.

For the filling

1. Throw all filling ingredients into the blender and blend for 30 seconds.
2. Transfer filling to a medium sauce pan and cook on medium-low heat, stirring continuously.
3. Cook until filling is thickened, about four minutes.
4. Remove filling from heat and let cool.

For the frosting

1. Beat butter in large mixing bowl until smooth and creamy.
2. Gradually add remaining ingredients until frosting is completely smooth and all ingredients are incorporated.
3. Once cupcakes are cooled, cut a small cone-shaped hold out of the top of the cupcake and fill the cupcake with banana cream custard.
4. Replace cupcake top and repeat for each cupcake.
5. Frost cupcakes and keep refrigerated until ready to serve.