Peel, chop, boil, drain, and mash potatoes. Let sit until cool enough to handle. (If you have leftover mashed potatoes in your fridge, you can use that and skip this step. Alternatively, if you have home-canned potatoes, they're already fully cooked, so you can drain them and mash them.)
Add the remaining ingredients to the bowl. If you're adding extra ingredients (see notes), also add them to the bowl.
Mix with a spoon until it starts to come together and is difficult to mix. Continue mixing with your hands. The dough should be slightly sticky; if it's too sticky you can add extra flour until it feels like a good bread dough consistency. Knead for a few minutes on a floured surface.
Divide dough into several small balls. Working with one ball at a time, roll dough out on a floured surface (or smash down with your hands if you don't have a roller) and slice into squares or triangles. You want it somewhere between a quarter and half a centimetre in thickness.
Put a pan over medium heat on the stove and add in olive oil and extra butter. Working in batches, fry until one side is golden and then flip and fry until both sides are golden. This should take a few minutes per batch and will speed up as you go along. You might need to add extra oil or butter between batches.
When a batch is done, move to a bowl or dish with a lid to keep it warm until all the batches are done. I like to line the bowl with a paper towel to absorb some of the extra oil and butter.
Best served warm.