Preheat oven to 400F. Brush the indentations of the madeleine moulds (or mini muffin cups) with melted butter. Dust with flour, tap off any excess and place in the fridge. Using a stand mixer with the paddle attachment (or hand-held with regular blades), beat the eggs, sugar, salt and vanilla for 5 minutes until pale and thickened. Beat in the lemon zest for the last minute. Combine the flour and baking powder. Using a spatula, gently fold it into the egg-sugar mixture in 3 or 4 additions. Gently fold in butter, a few spoonfuls at a time. Place batter in the freezer for 5 minutes. Fill each buttered and floured mould three-quarters full with batter, don't spread. Bake for 8-9 minutes or until the madeleines are just firm to the touch. Don't overbake. Meanwhile, in a small bowl, stir together lemon juice with enough icing sugar to make a smooth glaze about the thickness of tomato juice. Season with a pinch of salt. If too thick, add a little water. Remove the madeleines from the pan and cool on a rack. While still warm (but cool enough to handle), dip each one in the glaze, coating both sides and brushing off excess. Return to rack to dry, scalloped side up. Yield: 24, depending on the size of your mould pans.