To make the bechamel: Met the butter in a saucepan over low heat. Stir in the flour and cook, constantly stirring, until the flour has "cooked" for about 2 minutes. Slowly pour in the hot milk whisking constantly, and turn the heat to high. Continue whisking until the sauce has visibly thickened. Remove from the heat and salt, to taste. This recipe makes about 5 cups bechamel.
For the croquettes: Stir the cacopcavallo into the bechamel. Let the sauce cool completely. Cover and refrigerate the sauce at least 1 hour before preparing the croquettes.
Cut the bechamel into 2-inch chunks. Coat the chunks the eggs and then completely cover in bread crumbs. Place the croquette on a baking sheet to rest while you prepare the others.
Fill a deep pot no more than halfway with extra-virgin olive oil. Heat oil over medium-high heat until it reaches a temperature of 360 to 365 degrees F. The oil should remain at or around this temperature throughout the cooking process. Working in batches of 6, fry the croquettes until golden brown. Use a slotted spoon or spider to gently drop the croquettes into the oil, being careful not to splatter the hot oil. Using the same spoon or spider, remove the finished croquettes to drain on paper towels. Season with salt and serve immediately.