In a small saucepan set over medium heat, melt the butter, and stir occasionally. Once fully melted, remove from the heat and pour into a small bowl set aside to cool to room temperature.
In the bowl of an electric stand mixer fitted with the whisk attachment, place the eggs, sugar and lemon zest and beat on medium for 5 minutes until pale and thick. Decrease the speed to low and add the lemon juice and mix until combined.
Sift into the bowl the flour, baking powder and salt, add the poppy seeds and mix on low until a few flour streaks remain.
Remove the bowl from the stand mixer and slowly pour in the melted butter. Gently fold in the butter with the rubber spatula ensuring that you don't knock out all the air.
Cover the batter with cling film and refrigerate for 2-3 hours or overnight.
30 minutes before baking, pre-heat the oven to 180°C (350°F). Melt 15g / 1 tablespoon of butter and, using a pastry brush, grease all the moulds of your Madeleines pans. Lightly dust with flour and tap out any excess. Place the pans in the refrigerator until the oven has come to temperature and you are ready to fill them with the batter.
Once the oven is at temperature, remove the batter and pans from the fridge and scoop one level tablespoon of batter into the mould's deepest part. Repeat until all the shell moulds are full. Make sure that you don’t level off the batter in the pan. Scoop it in and leave it.
Place the Madeleine pans in the oven and bake for 8-10 minutes. I turn them at 8 minutes to ensure they brown evenly. Bake until golden on top, browned along the edges and risen in the middle.
Remove the pans from the oven and set them aside to cool for ten minutes. Give the pan a jiggle, and they should pop right out. Use a butter knife and run it gently along the edge if they don't come out when you jiggle the pan initially. Cool the Madeleines on a wire wrack.