Melt the chocolate pistoles at 45°C using a double boiler (Bain-Marie) or microwave (not recommeded).
Bring the smaller portion of cream to boil. If you wish to flavor the ganache, you can do it at this stage, and reheat the cream once properly infused, before adding it to the chocolate.
Add the hot cream gradually to the melted chocolate (in 3 to 4 times). Take the time at each step to stir energetically using the spatula with small circular movements in the very center. This will ensure the perfect and homogeneous incorporation of the cream into the chocolate (make an emulsion)*.
The ganache may be not very smooth after the first additions, but keep on stirring energically in the center, the edges will end up by being perfectly combined.
After the last addition, mix the ganache using the hand blender to get a perfect emulsion: your ganache should be smooth and shiny.
Add the bigger portion of UHT cream COLD and combine.
Cover with cling wrap directly on the surface and aloow it to stiffen in the refrigerator for at least 4 hours (ideally overnight).
Whip the ganache at low speed (like a chantilly) and use immediately.
