Grease the tray with butter and sprinkle with flour.
Place the butter in a bowl, add the sugar and salt. Rinse the orange in hot water, dab dry, grate the zest and squeeze the juice into the bowl. Mix using the whisk on a mixer. Whisk in the eggs and continue to beat until the mixture becomes lighter in colour. Mix the flour and baking powder, beat into the mixture.
Transfer the cake mixture to the prepared tray.
Approx. 20 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove from the tray and place on a plate.
Prick the madeleines firmly all over with a wooden skewer. Squeeze the juice from the orange, set aside 2 tbsp of the juice, mix the remaining juice with the icing sugar and pour over the madeleines.
Mix the icing sugar with the reserved orange juice. Dip the madeleines half way into the icing, leave to dry on a rack.