Melt the butter in a large skillet on medium heat. When the butter has melted, start adding handfuls of spinach. You may need to let some of it cook down just a bit before you can fit the rest of the spinach in. Just keep adding.
Sprinkle with salt and pepper and cook for three to five minutes, occasionally turning the wilted spinach on the bottom of the pan onto the top.
In the meantime, heat a serving plate. Spinach should be served (and eaten) piping hot. Top with croutons if you wish.