Grease a madeleine tray with melted butter and dust with flour. Tap out the excess flour.
Crush the cardamom pods using a pestle and mortar to release the seeds. Remove the shells and crush the seeds a little further.
Put the eggs, caster sugar, lemon rind and crushed cardamom seeds in a heatproof bowl and rest the bowl over a saucepan of gently simmering water. Whisk with a hand-held electric whisk until the mixture is thick and pale and the mixture leaves a trail when lifted.
Sift the flour and baking powder into the bowl and gently fold in using a large metal spoon. Drizzle the melted butter around the edges of the mixture and fold the ingredients together to combine. Spoon the mixture into the madeleine sections until about two-thirds full. (Keep the remaining mixture for a second batch.)
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for about 10 minutes until risen and golden. Leave in the tray for 5 minutes, then transfer to a wire rack.
Make the glaze by putting the lemon juice in a bowl and beating in the icing sugar. Brush over the madeleines and leave to set. Serve lightly dusted with icing sugar.