Wash the potatoes well, chop them into dices and let them rest in water for 5 minutes.
Add water to a pan, as if you were going to cook in a double boiler, inside this pan put a pasta strainer, a steamer basket or even a sieve.
Transfer the diced potatoes (drain the water) to the container inside the pan and cover. This will create steam to cook our potatoes.
If the water dries up, keep adding more until the potatoes are soft and smooth.
Remove from the double boiler, drain and transfer these potatoes to a bowl. Mash them well while still hot, with a fork or a potato masher. After mashing them, push this mash through a sieve using a spoon to make it nice and smooth.
Transfer the mashed potatoes to a saucepan, add the butter, the salt, black pepper and the corn starch. Mix well until combined, before bringing to the heat.
Turn the heat on medium/high temperature and stir nonstop until the potato mixture cooks through, for about 10 minutes.
Turn off the heat and let the mixture cool down.
Place this mixture on a flat surface and split it into 6 equal pieces, get one of the pieces and shape into a log. Repeat this procedure with all pieces of mixture and before slicing them, make sure they are not hot, let them cool down before you start slicing them.
Slice these logs into cubes and transfer them to a glass dish, add the sauce and serve.
If you want to store them, cover the dish with plastic wrap and serve whenever you want.