Start by preparing and baking your cupcakes according to box instructions.
Let cool completely.
Cut out center with a spoon, as shown and set top aside.
Beat egg yolks lightly and set aside.
Place sugar, corn starch, and milk in a pot, and whisk until well combined.
Place over medium heat and stir until bubbly.
Simmer for two minutes and remove from heat.
Take 1/4 cup of your mixture and add to your egg yolks.
Mix, and add back to pot.
Whisk until smooth custard forms.
Place back over heat and let bubble for two minutes until it becomes thick.
Remove from heat and add in vanilla and butter.
Cover and place in fridge until cool.
Fill the center of each cupcake with custard, and place the tops back on.
Pipe icing onto each cupcake.
Serve and enjoy!