3 cups seasoned mashed potatoes, chilled
2 1/4 cups plain dry bread crumbs
2 ounces Serrano ham cut into small dice (about 1/2 cup)
1/2 cup piquillo or roasted red peppers, cut into small dice
5 large eggs
1 large egg yolk
1 teaspoon freshly ground black pepper
3/4 teaspoon Spanish smoked paprika (piment?n)
1 cup all-purpose flour
Olive or vegetable oil for frying
Combine the potatoes, 3/4 cup bread crumbs, the ham, pepper, 1 egg, the yolk, a pinch of salt, 1/2 teaspoon black pepper, and the paprika, in a large bowl. Mix well.
Lightly beat the remaining 4 eggs in a wide shallow bowl. Place the remaining 1 1/2 cups bread crumbs in a second bowl, and the flour in a third. Season the bread crumbs with 1/2 teaspoon each salt and the remaining 1/2 teaspoon pepper.
Using about 2 tablespoons of the croqueta mixture for each, form it into 3-inch-long fingers. Dip each finger in the flour, tapping off the excess, then dip in the egg mixture, letting the excess drip off, and then in the bread crumbs. Transfer to a large baking sheet. When you have finished forming the croquetas, cover the tray with plastic wrap and refrigerate for at least 12 hours, or overnight.
When ready to fry, heat 1/4 inch of oil in a large skillet over medium-high heat. Fry the croquetas in batches, turning once, until dark golden all over, 2 to 3 minutes a side. Transfer to a paper-towel-lined plate and sprinkle with salt if desired. Serve hot.