Heat the oil in a heavy pot or Dutch oven on medium-high heat until it reaches 375°F on a digital instant-read thermometer. Then, turn the heat down to medium.
Meanwhile, cut the refrigerated mac and cheese into slabs roughly the size and shape of a deck of cards, approximately 1/2-inch squares.
Combine the flour, salt, pepper, and cayenne in a pie plate. Whisk the egg and water together in a second pie plate. Breadcrumbs go in a third pie plate.
Dredge each slab of mac and cheese in the flour and tap off as much excess powder as possible. Dip in the egg wash, then coat thoroughly with the breadcrumbs. Set aside on a cooling rack for 5 minutes to set. Ease 3 or 4 pieces into the oil at a time and fry until golden brown, 1 to 1 1/2 minutes per side, flipping once. Remove to a cooling rack set over paper towels. Repeat with remaining squares. Serve hot. I like mine with cold ketchup, but that’s just me.