Line a 10-by-15-inch rimmed baking sheet with parchment paper. In a double boiler, heat the chopped chocolate over simmering water, stirring frequently, until almost melted and an instant-read thermometer registers 105°, about 20 minutes. Remove the pan from the heat and stir until the thermometer registers 110°, 1 to
2 minutes more. Remove the insert from the pan, reserving the pot of water. Let the chocolate cool, stirring occasionally, until the thermometer registers 87°, 25 to
45 minutes.