Break the chocolate into small pieces in a bowl. Melt it in a double boiler.
Reserve off the heat. Add the butter in small cubes. Separate the egg whites from the yolks.
Beat the egg whites until stiff peaks form. Add the egg yolks to the melted chocolate, mix to obtain a smooth mixture. Incorporate the egg whites into the mixture, little by little and gently, so as not to lose the mousse texture.
Pipe the chocolate mousse into glasses using a pastry bag and refrigerate for 2 hours. Add the cherry syrup just before serving