For 12 ounces tempered chocolate, melt coarsely chopped semisweet chocolate in the top of a double boiler over simmering water until it registers 115 degrees to 120 degrees on an instant-read thermometer. Remove bowl from water. Let cool, stirring occasionally, until chocolate reaches 82 degrees to 86 degrees on an instant-read thermometer. At this point, it will begin to set around the bowl's edges. Return to the pot of hot, but no longer simmering, water for a few seconds at a time until it reaches 88 degrees to 91 degrees. Do not let chocolate become hotter than 91 degrees.