Here's the recipe I've been using for about 20 years now for my coconut cakes and chocolate cakes; it is awesome. It NEVER turns to sugar and you can eat it right out of the bowl and it is super!
White Marshmallow Frosting -
4 egg whites, room temperature
2 cups sugar
1/4 cup white Karo syrup
1/2 cup water
Cook sugar, Karo and water until it forms a hard ball. Beat egg whites in large bowl and beat until stiff. Pour hot syrup mixture slowly over beaten egg whites while beating. Beat continually. Pour in 2 cups miniature marshmallows. Beat until melted. Add 1 teaspoon vanilla (I use clear vanilla) and a dash of salt.
This makes a LARGE amount of frosting. For a regular size cake, I use 3/4 recipe which is:
3 egg whites
1 1/2 cup of sugar
3/16 cup (3/4 of 1/4 cup) Karo syrup
3/8 cup water
I really think you'll love using this recipe. I also like to make a white or yellow cake in the long metal pan and frost with this frosting then sprinkle coconut and pineapple chunks on top. It is very pretty and delicious.