Convenience stores sell these pre-made sandwiches.
There are two crustless sandwiches in a pack stuffed with various fillings.
Made by Yamazaki Baking Co and super cheap – around 100 yen each.
They also have seasonal and promotional varieties – like Fruits and Cream Frozen sandwiches:
You can get sweet ones full of cream, chocolate or peanut butter.
Also some meaty ones with yakiniku, katsu or croquette.
They have a long-shelf life, don’t need refrigeration and are perfect for a quick bite.
Check out their official site here.