Preheat your oven to 425° F (218° C). Grease a madeleine pan, dust with flour, and shake off the excess.
Melt your butter and set aside to cool slightly.
Measure your eggs and sugar into a large bowl and whisk until pale and thick. Note: Achieving this is much simpler with an electric mixer fitted with a whisk attachment. It is important that the the eggs are whisked enough.
Sift in half of the flour with baking powder, salt, and lemon zest. Fold them in gently with a flexible spatula.
Pour in half the melted butter around the edge of the bowl and fold in. Repeat the process with the remaining flour and butter.
Spoon the mixture into the madeleine pan (Note: you will have enough batter to fill your pan at least twice) so they are full to just below the rim. Bake 8-10 minutes, until well-risen and golden. Ease out of the pans, and cool on a wire rack.
Grease and flour your pan again. Fill your pan again, and bake your second batch.
Dust madeleines with confectioner's sugar, and serve.