Butter and flour Madeleine pan, making sure to get into all the little crevices.
Preheat oven to 375 degrees
In a bowl, beat the butter with a hand mixer or whisk until smooth, light and creamy.
In a large bowl, whisk the eggs, egg yolk and sugar until pale, thick and creamy. Add the clementine juice, zest and vanilla and continue beating.
Sift the cake flour and baking powder into the egg and sugar mixture and stir until incorporated.
Little by little, add the batter to the butter, stirring with a wooden spoon until all ingredients are incorporated. Cover and allow to rest in a cool place for about 20-30 minutes.
Using a small scoop (about a heaping tablespoon), scoop some batter into each mold and gently spread it out just a bit. It will expand during baking.
Bake for about 7-8 minutes, until edges begin to brown and they develop the characteristic hump.
Allow to cool for a couple of minutes before un-molding. Repeat with remaining batter.
When cool, dip Madeleines in melted dark chocolate. Allow the chocolate about 15 minutes to set up, then enjoy!