I recently posted a recipe for a "Wilton's" icing which is used by bakers but is not like marshmallow. Good though!
This is a recipe from "Company's Coming-Cakes" by Jean Pare. "A shiny soft white icing that resembles half melted marshmallows. It swirls prettily." Enjoy!

Seven Minute Frosting

2 Egg Whites
1 1/2 cups white sugar
1/3 cup Cold Water
1 1/2 teaspoons light corn syrup ( I use Karo)
1 teaspoon Vanilla

Put egg whites, sugar, water and syrup in top of double boiler. Beat until mixed well. Place over rapidly boiling water. Beat constantly with electric beater while it cooks for 7 minutes or until it will stand in peaks when beater is raised. Remove from heat. Add vanilla. Beat. Fills and frosts a 2 layer cake (8 or 9 inch).
Variation: Seafoam Frosting: Omit white sugar. Add 1 1/2 cups brown sugar and 1 teaspoon maple flavoring.