In a small bowl, combine the flour, baking powder, and salt. Stir to break up any clumps.
In a large bowl, beat the eggs, egg yolk, and sugar on medium-high speed until very pale yellow, forming ribbons, and more than doubled in volume.
Gently stir in the lemon zest and lemon juice.
Fold in the flour mix in four additions, incorporating gently and breaking up any pockets of flour without deflating the air too much.
Stir in the melted butter until fully incorporated.
Cover and allow it to rest in the fridge for at least 30 minutes (up to 2 hours).
While the batter is resting, preheat the oven to 375°F/190 C. Brush melted butter into a madeleine pan (or mini muffin pan). Dust with flour and pour out any excess.
When ready to bake, scoop a heaping tablespoon (15 ml) of batter into each well. No need to level it out; it will do so as it bakes.
Bake for 9–12 minutes until golden brown at the edges and bounces back in the center when lightly touched.
Allow to cool for 3-5 minutes in a pan. Use a paring knife to work around the edges, and pop the cakes out to cool completely. If you still have more batter and only one pan, clean the wells out and recoat with butter and flour to bake a second batch.
Once the cakes are completely cool, prepare a white chocolate coat.
Break the white chocolate into small pieces and combine them in a microwave-safe bowl with the coconut oil. Microwave for 30–40 seconds until starting to melt, then 15-second bursts until mostly melted. Stir to finish melting and create a smooth chocolate.**
If using liqueur, add it now, stirring gently.
Holding the madeleines at an angle, dip them half in the melted chocolate, shake off the excess, and set them on a cooling rack to firm up.
Alternatively, you can just drizzle the chocolate over the cakes in a zigzag.
It is best eaten the same day but can be stored in an airtight container in the fridge for 2 days.