Heat the oven to 180°C (160° fan) 350°F gas 4. Butter 24 madeleine tins.
Beat the butter and sugar in a mixing bowl until creamy. Beat in the egg until blended.
Stir in the flour, salt and rosewater. Cover the bowl and chill for 1 hour.
Spoon into the tins and bake for about 15 minutes, until golden. Cool in the tins for a few minutes, then place on a wire rack to cool completely.
Sift over a little icing sugar just before serving.