When melting chocolate, it is best to use a bain-marie, or a double boiler. This is a French invention, and it is simply a pan of hot or simmering water with a bowl suspended over it. It is used for making delicate sauces, custards and for melting chocolate, which direct heat would damage or bum. Only use a bain-marie if the bowl makes a proper seal with the pan of water underneath. If a small bowl is left to stand in the middle of a large pan of simmering water, steam may contaminate the chocolate, and even a small amount of water or steam ruins good chocolate. If you do not have a proper bain-marie, a useful addition to any kitchen is a large stainless steel bowl, available from any good cook ware shop or catering supply store, which works just as well. Placed over a pan of hot water, it is cheaper and more versatile than a bain-marie. Just make sure the bowl overlaps the sides of the pan, creating a good seal. In fact, all the chocolate chefs I have seen doing demonstrations favour this method.
When using a bain-marie or stainless steel bowl for melting chocolate, the water in the pan should not boil, and rarely needs to exceed 140°F (60°C). This is because most chocolate becomes molten at 113-131°F (45-55°C). Chocolate burns very easily and at surprisingly low temperatures. Milk and white chocolates are particularly susceptible, because they contain the milk protein casein, which denatures at 129°F (54°C) – even lower than the temperature at which some bittersweet (dark) chocolate melts.
It is possible to use a microwave to melt chocolate, but be very careful. Choose the lowest possible setting, and after an initial period of 30 seconds, use short bursts of 10 seconds, checking after each burst to see whether the chocolate has melted.
For those of you with a plate-warming oven, chocolate can be left to melt very slowly and gently overnight. However, you need to know your oven well to have the confidence to do this. Alternatively, chop the chocolate into squares and put into an ovenproof dish. Place the dish low down in the oven, set at the lowest temperature, and leave for about 15 minutes. The chocolate will retain its shape even when melted, so test it with a fork. I find this last method to be the simplest and best.
Other utensils which are useful for working with chocolate include a good balloon whisk and a couple of plastic spatulas.