Heat half of the cream (250mls) in a small saucepan over medium-high heat until bubbles begin to form around the edges.
Place the chopped chocolate into a heatproof bowl. When the cream is heated, pour it over the chocolate and add the vanilla. Leave to melt for a minute, then stir with a whisk until all of the chocolate is melted and the mixture is smooth and well combined.
Whisk in the remaining 250ml of cream. Adding this cold cream helps to cool the mixture down faster.
Taste the mixture, and if the chocolate is a little too bitter you can add some caster or icing (confectioner’s) sugar until it’s as sweet as you want it.Cover and refrigerate until very well chilled (at least several hours or overnight). If you're in a hurry you can chill it faster by putting it in the freezer. Just stir it regularly and be careful not to let it actually freeze.
Whip the chocolate cream with an electric hand mixer just until stiff peaks form. It won’t take very long, be careful not to overwhip it.
Pipe or spoon into glasses or small bowls. You can serve it immediately for a softer, smoother mousse, or chill for an hour or two for a fluffier mousse.
Store, covered, in the fridge for several days.
Chocolate: For best results, use good quality chocolate with 50% - 60% cocoa solids.Cream: The cream you need for this recipe is known by different names in different countries. Here in NZ it’s usually called standard cream or whipping cream, in other countries, it may also be known as single cream or full cream.Heavy cream (or heavy whipping cream) and double cream may be too thick for this recipe as they contain a lot more fat (although the fat content can vary by country.) If heavy cream is all you can get, be very careful not to overwhip the mousse, as it will thicken very quickly.Long story short: for best results, choose a pourable, un-whipped cream that is around 35% fat. It should say on the bottle/carton that it is suitable for whipping.