Place chopped chocolate in the top pan of a double boiler or in a heatproof bowl that will fit snugly in the rim of a heavy saucepan. Fill the bottom pan of the double boiler or the saucepan with water to a depth of about of 1 1/2 inches (4 cm) and heat over low heat until it barely simmers. Place the top pan or bowl holding the chocolate over (not touching) the water in the lower pan. Heat, stirring often, until the chocolate melts. Remove the top pan from over the water and set aside to cool slightly before using.