Marshmallow fluff makes a very sturdy cake filling, and the caramelization gives it a nice toasted marshmallow flavour!
1.5 cups sugar
6 tbsp water
1 tbsp corn syrup
2 egg whites (about 80 grams) at room temp
1/4 tsp cream of tartar
Place sugar and 3tbsp of water in a heavy bottom saucepan. Stir until all the sugar is wet, and place over high heat with a lid on (this helps prevent crystalization on the sides, although it’s slower, so if you’re confident with your caramel skills, just brush down the sides with a wet pastry brush like usual. It’s also easier to have a lid on if you have a glass lid).
Mix the remaining 3tbsp of water with the corn syrup, and get your 2 egg whites and cream of tartar ready in a stand mixer bowl.
Let the sugar become a deep caramel colour. Remove it from the heat*, and stir in the water/syrup mix. It will harden unevenly. Get your egg whites whipping as you want very frothy/maybe soft peaks before you start pouring the caramel syrup. Put the caramel back on the heat just long enough to have everything mix evenly. With the stand mixer running, slowly pour your hot caramel into the whipping egg whites (try to get the stream between the side of the bowl and the spinning whisk). Beat until it’s cooled down and a stiff but spreadable consistency. I didn’t add any additional flavours, but if you’re so inclined you could add some sort of extract at this point (I just wanted the authentic caramel flavour, and it really came through)
* I placed my saucepan in a bowl of cool water, I’m not sure if that was necessary to stop the caramelization because you instantly add in cool water, but that’s what I did