Chop two-thirds of desired amount of bittersweet or white chocolate into 1⁄4-inch pieces and place in a heatproof bowl, leaving the remaining one-third separate and in one piece.
Bring about 2 inches water to a simmer in a saucepan (the bowl holding the chocolate should not touch the water when set over the pan, so adjust the level of water as necessary). Lower heat and set chocolate over pan, stirring occasionally until smooth. Once melted, stir gently with a heatproof flexible spatula until candy thermometer registers 131 degrees (for bittersweet) or 118 degrees (for white chocolate).
Remove the bowl from the saucepan and add the remaining piece of chocolate to melted chocolate. Stir with a flexible spatula until it cools to 84 degrees (for bittersweet) or 81 degrees (for white chocolate). If necessary, remove unmelted chocolate and reserve for another use. Return bowl to saucepan and bring temperature back up to 86 degrees to 87 degrees for both types of chocolate. Use immediately.