I don't make soap - I figure there are people out there who have worked really really hard to create awesome soap and I'm happy to spend money on their products - so I use melt & pour. All the recipes for making icing for soap cupcakes required either making a cold process soap - not an option - or foaming bath butter - again, not an option (I can't find it locally and for a class, the cost is prohibitive). I have tried using cocoa butter or other butters whipped up, but they didn't feel right and reduced the foam on the cupcakes. So I thought about it and realized I could make a syndet icing like a shampoo bar.
25 grams SCI noodles or flakes
25 grams Amphosol CG (coco betaine)
35 grams SLSa
3 grams cetyl alcohol
2 grams fragrance oil (I used Cream Cheese Frosting from Brambleberry!)
Colouring (if desired)
Heat the SCI and coco betaine in a double boiler until the SCI has melted. Add the SLSa and cetyl alcohol and again heat until all ingredients are melted. Mix well with a hand mixer, then put into a piping bag with a cake decorating tip on the end (I used a 1M). Decorate cupcakes. Rejoice.
The reason I say not to quadruple the recipe is that it does set very quickly and you will find it difficult to pipe after about 20 minutes or so. If you want to decorate a ton of cupcakes, you will want to remake the recipe again.
You can use another liquid surfactant, but SCI melts best in an amphoteric surfactant like coco betaine, so this is why I suggest it. I used cetyl because it will harden and will offer some moisturizing - you can substitute stearic acid for a harder and slightly less moisturizing icing. Feel free to add some oils - say up to 3% - to this mixture for more moisturizing!
If you aren't into melt & pour soap, you could use this icing to make some decorations for your bath bomb cupcakes (although it's not going to melt quickly in the tub)!