Cook the potatoes starting from cold, salted water over medium heat. From the moment the water starts boiling, it will take 15-20 minutes depending on the size of the potatoes.
Strain the potatoes and leave them on the colander/sieve for a couple of minutes to let the excess water drain.
Use a potato ricer while the potatoes are still hot (it's a lot harder when they're cold) and add the butter. Mix. Wait for it to cool down before adding the cornstarch.
Add salt and cornstarch and mix with your hands until you have a homogeneous dough.
Divide into 8 portions and form balls. Flatten them out and fill them with cheese. Close them and form discs.
To cook them, preheat a non-stick pan over medium heat. Add a bit of butter and oil and cook for approximately 3 minutes per side or until golden.
Remove them and serve immediately.
If you need to reheat them, it's best to use an air fryer set to 180°C/350℉ for 10-15 minutes or until crispy again.