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.
Set up the blender to have it ready to use.
Pour the cold milk (550 g; 19.4 oz) 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 (208 g; 7.3 oz). Warm over medium heat, often stirring until the sugar dissolves.
Increase the heat to medium-high and bring to a boil (85° C / 185° F / when the cream bubbles up vigorously / if the cream starts to overflow). Remove from the heat.
Add the cocoa powder (45 g; 1.6 oz), whisking to dissolve it.
Return the saucepan to medium-high heat and cook briefly, constantly stirring with the rubber spatula, scraping the bottom of the saucepan to prevent it from scorching. Remove from the heat as soon as it starts to bubble up.
Pour the boiling hot cream into the blender jug with the cold milk.
Turn the blender on (medium speed). Note: by blending that much boiling hot milk with that much fridge-cold cream, 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.