Crap! I just lost everything I typed. Grrr....
If you don't have cocoa butter, you can use a small percentage in a batch. However, it will be softer than if you use CB and probably will require a slightly longer curing time.
Condensed version of what I wrote: I know a soapmaker who did use Palmer's Cocoa Butter in her soap. She sent me a bar and it was lovely. (I thought at the time I'd like to try making a batch but always forgot to buy some Palmer's.) She told me how she did it but it's been over 3 years so I don't remember exactly what she told me. I know she did say she didn't add a lot so I suggest trying it at 5% since it does have mineral oil. Too much mineral oil can make the soap feel greasy and since it doesn't saponify, it can seep if you have too much. Also, cocoa butter isn't very high on the list (4th ingredient). The first ingredient is Theobroma Cacao (Cocoa) Extract, mineral oil and then microcrystalline wax which from what I was able to find, seems to have been used as a thickener and to keep the other ingredients from separating. But there may be another function it performs.
Anyway, this is what I would do to calculate lye for the Palmer's. I would enter the amount I was going to use of cocoa butter into soapcalc to find out how much lye is recommended for saponification. Then I would use one fourth of the amount. I would reduce the amount of lye recommended for the entire batch by whatever amount it would take for the one fourth. Example: my recipe requires 128 g (5% lye discount). The amount of cocoa butter would be 1.6 oz for which 5.9 g lye is needed for saponification. One fourth of this amount would be 1.5 g so I'd subtract 4.4 g (5.9 g - 1.5 g) from 128 and which would be 123.6 g lye for the batch. I'd round it down and use 123 g. It doesn't seem like a big difference but I'd end up with a 1%-2% superfat by not reducing the lye. JMO but that's a little low for me since I generally use at least 5%.
I think the soap would be fine and not lye heavy by using the reduced amount combined with my usual lye discount.
Good luck if you decide to try it. Also, I'd like to hear how it turns out.