Melt butter in a large saucepan, whisk in flour, and cook for 3 minutes.
Slowly whisk in milk until the sauce is very thick and season with salt and pepper.
Stir in breadcrumbs, both types of ham, and parsley.
Chill the mixture in the fridge for 4 hours.
Shape the mixture into 7cm cylinders.
Coat each cylinder in flour, then beaten egg, and finally panko breadcrumbs.
Heat oil in a deep-fat fryer to 170°C.
Fry the croquetas in batches until golden-brown and drain on kitchen paper.
Whisk egg yolks and mustard while gradually adding oil to make mayonnaise.
Stir in vinegar and garlic, then season with salt and pepper.
Serve croquetas with the garlic mayonnaise.