Melt the butter over low heat. Mix the eggs with the sugar. Add the orange zest. Mix the flour with the baking powder and a pinch of salt. Add the orange liqueur to the egg mixture and beat until you obtain a light, airy texture. Stir in the melted butter. Fold in the flour mixture. Mix until you obtain a smooth batter. Refrigerate for a few hours. Pre-heat the oven to 180°C. Spoon the batter into a madeleine pan and bake for 10 min.