In a bowl, mix the mascarpone and sugar.Add the lime zest and juice.Beat the egg whites until stiff and incorporate them into the first mixture.Pour into glass cups at an angle and fill halfway. Refrigerate for 2 hours before serving.Break the chocolate into small pieces in a bowl. Melt the chocolate in a double boiler.Off the heat, add the cubed butter.Separate the egg whites from the yolks. Beat the egg whites until stiff. Add the egg yolks to the melted chocolate, mix to obtain a smooth mixture.Incorporate the stiff egg whites into the chocolate mixture with a spatula, a little at a time, to avoid losing the mousse texture. Remove the cups from the fridge and fill them completely with the chocolate mousse over the lime mousse. Refrigerate for 1 hour before serving.