Combine 3 cups of the milk, the sugar, and egg yolks in a large saucepan. Cook and stir in medium heat until mixture just coats a metal spoon. Remove from heat. Stir in melted chocolate. Stir with a wire whisk or beat with a rotary beater until smooth. Stir in remaining milk. Cover surface with plastic wrap. Refrigerate several hours or overnight until completely chilled. (Or, place the saucepan in a sink of ice water to chill quickly.)
Stir in chopped chocolate. Freeze in a 4- or 5-quart ice cream freezer according to the manufacturers directions. Makes about 2-1/2 quarts (20 servings).