This seems like a long shot, but worth asking. I have a fragile fragrance oil that won't survive CP, and I'd really love to make soap cupcakes with it.
If I pour HP cupcake bases with the fragile scent, I could add CP icing, but is there a way to make the CP frosting stick to the HP cake layer? Seems like HP instantly cools into something very non-sticky.
Maybe the CP icing would stick if I spray the HP with distilled water to get it damp?
I've seen videos of HP frosting, but it's not beautiful. I doubt HP would do a better job of sticking to itself.
I'd do this:
1. Cover the HP cupcakes with plastic wrap to hold the moisture content immediately after pouring into the mold(s). Keep covered until right before piping the tops.
2. Score the tops of the HP cupcakes. Use a knife to create criss-cross lines to give the frosting little crevices to hold onto. The scores don't need to be deep, just enough to provide a bit more for the new soap to grab hold of. If you plan ahead, you can score immediately after pouring into the molds (or wait until later.)
3. If the HP cupcake seems too dry, brush lightly with warm or hot water, but only very lightly; you don't want a runny watery mess on top.
4. Use a little leftover soap batter saved in the batter bowl (cover with plastic wrap to keep it moist until use) to paint the scored surface immediately prior to piping the CP icing.
5. Depending on the recipe & firmness of the tops, I may try CPOP in pre-heated oven (low temperature) to weld them together. (Click on the highlighted link below for more information on this process.)
Whether or not you feel comfortable CPOPing piped soap, it is another step you could take that I have done to ensure adherence of layers of soap when new soap is added to an older soap. I do know some folks have CPOP'd piped topped soap to encourage gel and had no problems, but I think it's fairly uncommon as I don't see it mentioned much. Most recently I saw it discussed on a reddit soapmaking thread.
I have successfully welded soap together many times using these methods (sometimes with the criss-cross roughening, and sometimes without). I find that wrapping in plastic wrap to retain moisture of the old soap (need to plan ahead for this part of it, of course), works great when I want straight lines at the 'weld'. (Link
on this method.) But I have not done it with piped tops, so I'd try it to see how it works.
if you decide to use the CPOP method to complete the weld: Allow the soaps to completely cool off inside the oven or on a tray removed from the oven and placed on a counter out of the way where they will not be touched for several hours. DO NOT TOUCH or pick up the soap while it is still hot because you may squish the soap and change it's shape or leave an indentation. Take my word for it - Do not touch until completely cooled.
I look forward to hearing your report of what you do choose to do and how it works for you and your soap. And pictures, too!