According to "Hotel Chocolat" recipe for the "caramel" bar, there is barely 10-15% of caramelized sugar. I tried my caramel milk chocolate recipe:
20% cacao nibs
28.6% cacao butter
15.4% caramelized sugar
27% skim milk powder
9% Cashew nuts
0.37% lecithin

What I found is just a slight hint for caramel flavour.
I carmelized the sugar on a pan, the color changed to caramel color and the temperature was higher than 160C. By all parameters it was a good caramel.
I also think to caramelized the milk powder but its time consuming and the flavour is approximately the same (according to previous experience).
I is noteworthy to mention that I mix 50% of Hotel Chocolat caramel with 100% dark chocolate and I could still sense the caramel.

So, What is the secret?

I am very confused about what are you actually asking about?

How to make chocolate with with caramel flavour/taste?

This can be achieved in the process of making chocolate with normal milk chocolate recipe. Best method is to keep the mass at dry stage at 75-78C for 1-2hours.   

Yes. Precisely.
You mean making regular milk chocolate and then heat it to 75C? it will not burn the chocolate? I suppose that this process caramelized only the milk powder because caramel stage in sugar is higher than 160C.

You can develop a very strong caramel flavour in milk chocolate if during chocolate making process.

Typically you would want to keep the mass temperature below 55C for first 2 hours to remove the moisture and then raise it to 75-78C for 2 hours while it is still dry/pasty. Drop down to 65C and continue normal way. All before liquifying the chocolate.

Trying to do the same with already made/finished chocolate will not work.

thank you,
I didn't fully understand the process..
I just put the cocoa liquor in my Santha 11 and let it grind. Undoubtedly, the temperature inside can not rise over ~60C because of the epoxy.
I can put the chocolate in the oven but what does it mean "before liquifying the chocolate?"

Hi,

Please understand that I work in an operation making tens of tons of chocolate every week and I can offer very limited advice on how to use small scale machinery.

I am not sure if you can replicate "normal" chocolate making process in a Santha wet grinder and putting chocolate in the oven will not work.

"Normal" as what what is used to make most of the chocolate worldwide, using mixers, refiners and conches.

Process where raw materials are being dosed and mixed, refined to required particle size and loaded into chocolate conche as dry flake.

This flake in the conche is processed "dry and powdery" first to remove moisture, turned into "dry and pasty" phase to allow mechanical work and flavour development and at the very end cocoa butter and lecithin are added to liquefy mass and turn it into liquid chocolate with correct viscosity.

By controlling a range of variables like temperature, time, speed and direction of rotors or level of ventilation in the "dry and pasty" stage we can create a very wide range of chocolates from the exactly same mix of raw materials.

Conching is the magical part of chocolate making as we have a good idea how to do this but very little understanding of what exactly is happening.

Finding the exact explanation of what is happening during conching is not very easy as we have very limited understanding of that process.

It needs to be pointed out that we only measure "average" temperature of the mass in the conche using a relatively basic temperature sensor. Mechanical action of conche tools may result in "local" temperatures much higher (I have been told as high as 2,000C at sugar crystal breaks)  than one indicated so the answer to your question of what is happening is probably: all of the above reactions and some more. 

Source of 75-78C is experience, less than this and we get very limited caramelisation, more and caramelisation is difficult to control. Changing the temperature of chocolate mass in the running conche is not a very fast process.