Chill a large bowl. Melt the chocolate in the top of a double boiler over simmering water or a small bowl set over a saucepan of just-simmering water. Remove from the heat and let the chocolate cool, but don't let it harden. Transfer the softened ice cream to the chilled bowl. While folding it with a rubber spatula, drizzle the melted chocolate into it through the holes of a slotted spoon. Transfer the ice cream to a storage container and freeze until firm. Store for up to 2 weeks in the freezer.