Melt butter in a large saucepan, whisk in flour, and cook for 2-3 minutes.
Gradually whisk in milk to form a thick sauce and season with salt and pepper.
Stir in the fresh breadcrumbs, ham, cheese, and parsley then chill for 3-4 hours.
Shape the chilled 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.
Deep fry the croquetas in batches until golden-brown and drain on kitchen paper.
Whisk egg yolks with mustard while gradually adding oil to create the mayonnaise.
Stir in vinegar, season with salt and pepper, and serve with the croquetas.