Storage: Tart will keep for up to three days at room temperature, well-wrapped.

Troubleshooting: If any bits of caramel remain after adding the butter, simply stir the mixture over very low heat until they’re all completely dissolved. If any stubborn little bits remain, you can strain the buttery caramel through a mesh sieve, before adding the chocolate.

Unsweetened chocolate is often labeled as 99% or 100% cacao, or bitter (as opposed to bittersweet) chocolate and it’s easy to find at any supermarket in America. In France, it’s called pâte de cacao. (In Paris, you can find it at G. Detou, in bulk.) If it’s not possible to find it where you live, you could likely get away with just using 6 ounces (170 g) of the darkest bittersweet chocolate you can find.