Preheat oven to 200C/400F. Grease 20 holes of two 12-hole madeleine pans with 1 tbsp of the melted butter. Dust with a little flour and shake away the excess.
Beat eggs, sugar, rind and vanilla in a small bowl with an electric mixer for about 5 minutes until thick and creamy.
Sift flour and baking powder twice; fold into egg mixture with the remaining butter. Drop tablespoons of mixtures into pan holes.
Bake madeleines for about 10 minutes or until golden and firm to touch. Stand in pans for 2 minutes; transfer to a wire rack over an oven tray to cool.
Meanwhile make the glace icing. Sift icing sugar into a small heatproof bowl; stir in butter and enough juice to form a paste. Place bowl over a small saucepan of simmering water; stir until the icing is a pourable consistency. Do not overheat.
Spoon icing over madeleines and stand for 5 minutes or until set.