Tough to do. I haven’t seen a commercial cacao supplement that really competes with Lindt bars… and I’ve thought about manufacturing one. [1] A placebo with 0% cacao and competitive sensory qualities seems out of reach.
[1] Not sure what the active agent is, it might even be the fat that I’d want to cut out
The scientific approach is to assess their responses after giving them dark chocolate and dark chocolate placebo, double blinded.