In a medium saucepan heat milk. In the meantime combine sugar with cornstarch. Add slowly to the milk and whisk vigorously. Remove from heat when it will become thick. Add vanilla extract and butter. Combine well. Cool off completely. You can also put in the fridge to speed up the cooling time.
Preheat oven to 350 F. Prepare muffin baking pan with liners.
In a medium bowl combine flour and baking soda. Set aside.
In a separate bowl beat butter with sugar until smooth with electric mixer. Add eggs, each at a time and mix. Add vanilla extract, mashed banana and sour cream. Mix everything just to combine.
Add dry ingredients to the wet batter and just mix all ingredients until well combined.
Pour the batter into the liners (about 3/4 high).
Bake for 20 - 25 minutes until inserted toothpick will come out clean.
Remove from oven and cool off on wire rack.
In a medium bowl mix whipping cream until soft peak with electric mixer.
Add powdered sugar.
Pour about 1 Tbsp. pudding on the top of each cupcake. And finish frosting with whipped cream.