Melt butter in a saucepan, whisk in 200g of flour for 3 minutes, then slowly whisk in milk to make a thick sauce.
Divide sauce into two bowls, adding ham, manchego, and breadcrumbs to one, and cooked salt cod to the other.
Chill both mixtures in the fridge for 3 to 4 hours.
Heat rapeseed oil in a deep pan to 170°C.
Shape chilled mixture into 7cm cylinders and coat sequentially in flour, beaten eggs, and dried breadcrumbs.
Deep-fry croquetas in batches until golden-brown and drain on kitchen paper.
Blend aioli ingredients, except olive oil, in a food processor until smooth.
Gradually stream in olive oil while blending until a thick, yellow sauce forms.
Serve the croquetas on a board with the aioli for dipping.