Bruise the cardamom pods. Separate the medium eggs. Break the dark chocolate into small pieces. Stone the cherries.
To make the mousse, steep the cardamom pods in 1 tbsp of boiling water per pod for 10 minutes.
Strain the water into a heatproof bowl and discard the cardamom pods. Add the chocolate.
Sit the bowl over a pan of gently simmering water and stir occasionally, until the chocolate is melted. Remove from the heat.
Beat in the egg yolks, one at a time, followed by the butter.
Whisk the egg whites in a clean bowl until stiff, then gradually add the sugar, whisking between each addition.
Fold into the chocolate mixture and spoon into 6 dessert glasses. Chill for at least 2 hours.