Heavy (double) cream, 1½cups (12fl oz/375ml), chilled
Powdered (icing) sugar, ⅔cup (2½oz/75g)
Instant coffee powder, 1tablespoon
Pure vanilla extract, 1teaspoon
Melt the chocolate
Put the chocolate in the top of a double boiler and place it over (not touching) barely simmering water. Heat, stirring often, until the chocolate melts. Scrape the melted chocolate into a large bowl and set aside.
Whip the cream
In another large bowl, using an electric mixer on medium-high speed, beat together the cream, powdered sugar, instant coffee powder, and vanilla until firm peaks form.
Finish the mousse
Beat about one-third of the whipped cream mixture into the chocolate until smooth. Using a rubber spatula, fold the remaining whipped cream into the chocolate mixture. Transfer the mousse to footed goblets or bowls. Cover and refrigerate for at least 2 hours or up to overnight. Serve chilled.