We use the bain marie technique (double boiler) when we want to melt chocolate. Chocolate is sensitive to heat and burns easily. With the bain marie technique is is easier to make an egg based cream (egg doesn’t take well heat too).
The double boiler technique is as follows: we boil water in a very large pot and inside that pot we put a second, smaller, pot which is heated by the steam. In our smaller pot we melt or mix the needed ingredients for our recipe.
Note: be carefull not to drop by accident any water in the pot where you melt your materials.