A double boiler is made up of two containers. One is nested on top of the other to facilitate indirect cooking. This is called the double boiler method and is used to cook delicate ingredients such as melting chocolate, curds, and custards.
A double boiler is also called a 'bain marie' in French. It is a very common indirect cooking technique used in French cooking.
You have probably used it when melting chocolate, cooking Creme Anglaise, and sabayon, as well as when preparing hollandaise sauce and butter or cream-based gravy. It is not only used by cooks but also by soap makers and candle makers to melt the beeswax.
You can buy a commercial double boiler in the market but honestly, you can also make it yourself with dishes you already have in your pantry. I used my saucepan and heat bowl pyrex glass bowl.
Take a pan like a soup pot or sauce pan with little water in it and bring it to a boil. Reduce heat to a gentle simmer
Find another pan like a heat-proof mixing bowl that sits comfortably over that first pot
The top pot must not touch the simmering water in the bottom pot.
There should be no gap between the two pots to prevent any steam from escaping.
This is your DB. Water on the bottom and delicate ingredients such as chocolate in the top pan. 
It is quite simple really you put water in the bottom pan and ingredients in the top pan. As the water heats up the steam gets trapped in between the two pots. This creates enough heat to cook the delicate in the top pan.
With indirect cooking, the steam from the bottom pan heats the ingredients in the top pan. Is it important that the water in the bottom pan does not touch the top pan as it can cause delicate ingredients like chocolate to burn? 
It is very commonly used to melt chocolate and create chocolate treats as well as cook eggs. For example when tempering eggs to use in custards, sauces, frostings, etc.
The video shows me tempering first the egg yolks with sugar, then the egg whites with sugar, and finally the chocolate with butter over a double boiler. I used these to make my chocolate mousse.
They are both indirect cooking methods. But, in a double boiler, the second pan does not come in direct contact with the boiling water. While in a water bath the second pan is placed in direct contact with the boiling water. We used the water bath method to cook creme brulee, creme caramel, and flan.
Tips when working with a double boiler.
- Use the appropriate two containers that will nest into each other properly to prevent any escape from hot steam that can cause serious burns.
- Always use oven mitts or gloves to prevent steam from burning your hands.
- The water in the bottom pan must not be boiling water but rather simmering gently.
- Use a rubber or silicon spatula when working with delicate ingredients.
- Keep the burner flame to medium or medium-low to bottom of the pan from becoming too hot. High heat can cause all the water to evaporate quickly.
- Cooking delicate ingredients can be quite a messy job because chocolate can seize and eggs can curdle. So, be patient.
Auta Vaz Barretto
Thank you so much I learnt a lot from your classes