Let the chocolate melt. You can do this - as it really should be done - au bain-marie, but I personally put the chocolate in a bowl in the oven at 50°C. That also works fine and is easy. Mix the cream with the mascarpone and add the cocoa powder. Then pour in the milk, possibly a few drops of vanilla extract, and mix in the chocolate. Divide among the bowls.

Tip: portions of chocolate mousse
I like to serve the mousse as small desserts and then divide the amount into six or eight servings over three days.

How to make chocolate ice cream.
Put the chocolate mousse in a plastic container (a storage container) and put it in the freezer. Set your timer and stir every 20 minutes to loosen the ice crystals. After 1 to 1.5 hours, you will have ice cream. Serve immediately. Stirring is necessary. If you don't, you will end up with one hard ice cube.
If you have leftover ice cream, store it in the freezer. Take it out 1.5 hours before you want to eat it and put it in the refrigerator. After that, the ice cream will be soft and you can use it. I cut some rose petals to go with it as decoration.

Tip: sugar
I personally did not add sugar to this recipe, because chocolate with 70% cocoa already contains about 30% sugar. So there is indeed sugar in this dessert, maybe about 20 grams (depending on which chocolate you use). For me that's enough, chocolate doesn't have to taste really sweet. But to everyone, of course, the freedom to add extra sugar here.