In a large heavy-bottomed saucepan bring 5 cups water and salt to a boil then reduce to a bare simmer. Add polenta in a thin stream, stirring constantly; it will take about 10 minutes to add it all and will be fairly thick. Cook over low heat, 20 minutes, stirring constantly. Add melted butter, 4 tablespoons of the mascarpone, milk and nutmeg. Stir well.
Pour hot polenta mixture into a very wide, warmed serving bowl. Top decoratively with mascarpone in 8 heaping spoonfuls. Serve immediately. Pass pepper mill and a bowl of extra mascarpone, if desired.