Melt 1 1/2 pounds chocolate in top of a double boiler (or clean metal bowl set over simmering water). Remove chocolate from heat when it reaches between 110 and 115 degrees F on a candy thermometer. To bring into tempering range, add remaining chocolate to the mixture and stir constantly with a rubber spatula until the thermometer registers 82 to 84 degrees. Put chocolate back over the hot water and bring temperature to between 88 and 91 degrees. Be careful not to let the temperature fall too far or you will have to gently re-warm the mixture; if the temperature rises above 91 degrees in the tempering step, you will have to start the process over. Once tempered, use immediately for dipping. Store tempered chocolate at room temperature.
Recipe courtesy of Mark Bittman