This is how I made my croquettes.
1 egg yolk
1/4 cup flour
1/2 cup grated cheese
a few sliced of chopped ham
1/4 cup flour extra
1 T milk
1/2 cup breadcrumbs
oil for deep frying
Boil potatoes then mash together with egg yolk, butter, flour, cheese and ham. Season with salt and pepper. Recipe said to cover and refrigerate at this stage, but I didn't. Using floured hands shape handfuls of the mixture into cylinders about 2cm thick and 8cm long (but really any shape or size would be fine). Dip the croquettes in combined egg and milk then roll in the breadcrumbs (I used panko crumbs which go extra crispy and crunchy). Again the recipe said to refrigerate at this stage but I didn't. Heat oil in a wok, saucepan or deepfryer and deep fry the croquettes in batches until browned lightly. Drain on absorbent paper.
I made 20 croquettes with this amount of mixture.
A variation would be to mix some pesto into the dough and mould the croquettes around a small square of mozzarella.