Rather pentobarbital or secobarbital. As far as I know these are used for euthanasia (15g) and should be pretty nice (you fall asleep and die unconscious). However, they are apparently quite bitter.

Although… ice cream. You probably don’t eat that in one go like a glass of water, so the dose may not be high enough before you fall asleep. Secobarbital was used to treat insomnia in the past, by the way.