Gabriel Boudier Creme de Cassis, about 1 part to 5 or 6 parts of fizzy water, with a slice of lime.
This cassis, made by the maison of Boudier, was bought for the label. It's beautiful, and apparently faithful to its 1874 incarnation.
I usually use cassis for Kirs (a slosh of cassis into dry white wine), but to taste it well I just mixed it with cold bubbly water - it is 40% proof.
I also realized, when I did taste its rich syrupy black currant essence, that I have not treated my perennial stock of cassis well: it must be refrigerated or it oxidises after few months. That would explain the brown edges in the decanter. It is a shame to keep this beautiful bottle in the fridge but I'll bring it out for parties.