My sister developed this recipe to make a cake filling for a lemon wedding cake. She then used it as a topping(instead of frosting) for a gluten-free cake she made for my daughters and I to try. It's delicious! And a very nice change from buttercream frosting.
First, have cake baked and cool.
Prepare lemon sauce:
Heat 1/2 cup water, 1/4 cup lemon juice, and 1/8 tsp. salt in saucepan on stove, just before simmering. In separate bowl, add just enough water to 1 Tablespoon cornstarch to form a paste. Pour heated water/lemon mixture into cornstarch paste, stirring. Transfer back to saucepan, stirring on medium heat, about 1 minute, until mixture is transparent. Cool while making marshmallow topping.
Prepare marshmallow mixture:
Combine 1 c sugar, 1 Tablespoon gelatin(unflavored), and 1/2 c. water in saucepan. Heat and stir just until sugar is dissolved. Stire in 1/2 tsp. vanilla. Cool about 5 min. Transfer to electronic mixer-beat high speed 10 minutes.
When marshmallow mixture is ready(when mixture falls from beaters it keeps a soft shape on the mixture below), gently fold in the lemon sauce. If using lemony-marshmallow as filling, pipe buttercream frosting(can be non-dairy) around edges of single layer cake. Fill in with marshmallow mixture. Then stack next layer of cake on top. If using as a topping, see if the mixture is set up enough to stay on cake.
You may want to wait awhile for mixture to set up before "frosting" the cake. If you wait too long, it will become too thick, however. For me, I waited probably an hour, and mixture was set up enough that there were chunks in the "frosting" when I spread it on the cake. These lumps did come out, though, with enough spreading. Some of the mixture slumped at the bottom, but I scraped it off and smoothed it over. This is still an experimental recipe, but turned out looking really good on the cake, and was refreshing and delicious!