Before starting, read Get to know your blender in the questions and troubleshooting section below.
Place a rubber spatula and a whisk on a plate next to the stovetop to have them ready to use interchangeably.
If you haven’t already done so: melt the chocolate in a heatproof bowl and over a saucepan with simmering water; the bottom of the bowl should not touch the water.
Set up the blender to have it ready to use.
Pour 240 g of the cold milk (8.5 oz; 1 cup) into the blender jug (or a large bowl, if using an immersion blender). If you do not proceed with the recipe immediately, put it in the refrigerator to keep it cold.
Bring the heavy cream to a boil: in a medium saucepan, put the heavy cream (395 g; 13.9 oz) and the sugar.
Warm over medium heat, stirring often to dissolve the sugar. Increase the heat to medium-high; remove from the heat when it comes to a boil (this is at 85° C / 185° F / when the surface is covered with bubbles which pop vigorously / if the cream starts to overflow).
Pour the boiling cream into the blender jug with the cold milk.
Turn the blender on (medium speed). Note: by blending that much boiling hot cream with that much fridge-cold milk, the blend instantly reaches the right temperature for the xanthan gum to dissolve efficiently.
Sprinkle in the xanthan gum: with the blender on, sprinkle the xanthan gum (½ teaspoon) over the surface and blend for 2 minutes to fully hydrate the xanthan gum. Do not expect the blend to thicken; it will thicken as it cools.
Add the melted chocolate: stop the blender and pour a generous splash of the blend into the melted chocolate. Stir to loosen the melted chocolate, then pour everything into the blender, scrapping along any residues from the bowl.
Blend to combine.
Add the rest of the cold milk (310 g; 10.9 oz; 1⅓ cup) into the blender and blend until the mixture is a uniform brown colour with no streaks.