Cut the hanpen into bite-size cubes (we cut into 36 pieces) and place in a medium bowl.
Remove the mentaiko from the thin sac/membrane Discard the sac (or eat it as a snack!) and place the mentaiko into the bowl. *There are many ways to remove the sac/membrane:- Use one hand to hold one end of the mentaiko and at the same time use chopsticks to press and squeeze all the roe out into a small bowl.- Or use a small knife to cut a slit into the sac/membrane and scrape out all the roe. Add the Kewpie mayonnaise. Use a spoon to gently mix everything together.
Divide into cupcake liners and bake in the toaster oven (or regular oven) at 400F for 5 minutes.