Sift together the flour, cornflour, baking powder and salt. Set aside.
In a separate bowl, whisk together the melted butter, caster sugar, yogurt, milk, vanilla extract and lemon zest until very well combined.
Tip the dry ingredients into the wet and stir until no dry patches remain but do not over-beat the batter.
Cover the bowl and place it in the fridge to chill for at least 2 hours. This step is very important so don’t skip it. The batter will keep in the fridge overnight if needed.
Brush your Madeleine tray(s) with melted vegan butter. If you don’t have a really good non-stick one then you will also need to dust it with flour and tap out the excess. Place the tray in the fridge to chill along with the batter.
Once the batter has chilled, place a dollop of it in the middle of each depression of the prepared tray. The exact amount of batter that you need is going to vary depending on your tray, so you may want to make a test batch to work it out exactly, but about a tablespoon was the right amount for me. You don’t need to spread the batter out, but try and make sure that it follows the shape of the mold, rather than just sitting in a round blob. Return any unused batter to the fridge.
Return the tray(s) to the fridge for twenty minutes to chill while you preheat the oven to 240°C/220°C fan/450°F/gas mark 8
Bake the Madeleines in the hot oven for about 4-5 minutes, until they have formed their humps and are golden around the edges; then turn the oven down to 180°C/160°C fan/350°F/gas mark 4 and bake for another 2-5 minutes until a toothpick inserted into the centre comes out clean.
Let them cool for a couple of minutes, then carefully turn them on their sides in the tin and leave them to cool completely. Placing them on a wire rack may squish the hump or mark the shell pattern which is why they are cooled like this instead.
If you only have the one Madeleine tray and still have batter in the fridge then you will need to wash and re-grease the pan, then pop it in the freezer for 10 minutes to chill before filling it with the remaining batter, chilling for another 20 minutes and baking the rest of the Madeleines as above.