Melt the chocolate and butter in a thin-sided bowl over the just-boiling bain-marie, allow to cool a little. Stir in the egg yolks. Beat the egg whites with the salt until stiff. Add the sugar, continue beating until the egg whites turn glossy, carefully fold into the chocolate mixture, divide into the prepared ramekins, level off, individually cover with foil and freeze for approx. 4 hours. Remove the ramekins and place on a cloth in an oven-proof dish.
Cook in a bain-marie: pour in boiling water to 2/3 the height of the ramekins. Cook the chocolate cakes for approx. 35 mins. in the lower half of an oven preheated to 180°C. Remove, dust with icing sugar.
Mix the melon balls with the basil.