Lindt do both a 70% & 85%... but you have to be extra careful when trying to melt it - it can separate, etc.

Get a pan of boiling water, over the top of that put another (coverable, preferably), insert the chocolate broken up into smallish pieces, then remove the boiling pan from any heat source.

The amount of heat in the system will be sufficient to melt it.

Do not let any water, or any cold liquid such as cream come in contact with it - it will curdle or separate.

But why you would want to contaminate such hard to obtain expensive chocolate with such things, is like adding water or ice to an expensive single malt whiskey...