Drain the potatoes. While hot, mash the 4 tbsps. of butter. Season with salt and pepper. Cool.
To the cooled mashed potatoes, add the panko, parsley and egg. Mix.
With floured hands, form the mixture into patties about three-quarters of an inch thick and three inches in diameter.
Heat enough vegetable oil and butter, in equal amounts, to lightly coat the bottom of the frying pan.
Over high heat, fry the patties, a few at a time (how many depends on the size of your frying pan) until the underside is nicely browned.
Flip carefully and brown the opposite side.
Add more vegetable oil and batter, fry the next batch, and so on, and so forth.
Serve the mashed potato pancakes as a side dish or a snack.