Homemade marshmallow sauce can be used as a topper for ice cream or an ingredient. To make Rocky Road, Phish Food, Marshmallow Fluff Ice Cream or S'mores Ice Cream, you would add this in the last 5 minutes of making the ice cream by folding it in.
Add the unflavored powdered gelatin to the mixing bowl of an electric mixer.
Add ice-cold water and stir to dissolve the gelatin, then set aside (adding hot water will activate the gelatin and make it rubbery and hard).
In a heavy pot over medium-high heat, add the sugar, corn syrup, and a pinch of salt.
Bring the sugar/syrup mixture to a boil, then keep it at a gentle boil until it reaches 240 degrees.
Start the mixer on low speed and add the hot sugar/syrup mixture. Increase speed and whip until white marshmallow sauce forms. This will take a few minutes on high.
Once the desired thickness of the sauce has been reached, add 100% pure vanilla and whisk briefly.
Serve right away or store in a jar (once cooled) with a lid in the fridge for up to a month. Use the marshmallow sauce wherever you would use store-bought, especially when making ice cream or ice cream parfaits.
Notes
I've used this Marshmallow Sauce mainly in homemade ice cream or on brownies.