In a large sauce pan melt the butter. Do not let it burn.
Keeping the flames at medium-low. Add the flour a little at a time as you keep stirring the melted butter. This will form into a dry dough.
Then slowly pour in the milk, not quit using the entire cup. Pour a little of the milk at a time into the pan while continuing to stir. Each time you add milk keep stirring till the dough pulls away from the sides of the pan and forms into a large shiny ball. Once you have the right consistency remove from stove.
Drain your can of meat or fish and add into the dough, making sure it is distributed evenly.
Place dough in a bowl, cover with plastic-wrap and refrigerate for at least 2 hours or overnight.
Scramble the eggs in a bowl and pour a cup or more of breadcrumbs into a separate bowl.
In a large frying pan heat enough oil to cover the bottom of the pan well.
Take a piece of dough no bigger than the palm of your hand and roll it into a shape of your choice. Take your little shaped dough and dip it into the egg bowl & then into the bread crumbs bowl to coat evenly.
Fry each croqueta on all sides evenly until brown. Place fried croquetas on a plate with paper towels to drain some of the oil off.