Brush Madeleine pan cups with melted butter and dust with flour.
In a heat proof bowl, combine eggs and sugar. Place bowl over pan of simmering water. Beat until mixture is thick and pale yellow. Remove from heat. Continue to beat until cooled and slightly increased in volume.
Add flour, 6 oz butter and rind. fold lightly and quickly until ingredients are just combined.
Spoon into prepared molds. Bake 10-12 minutes.
Cool. Dust with confectioners sugar if desired.