1. Firstly melt the butter on a medium heat until the foam subsides and it's starting to turn golden brown. Quickly pour into another dish so it doesn't colour further, and allow to cool while you start making the madeleine batter.
2. In a large bowl, beat together the eggs, sugar and vanilla extract (if using) for 5 minutes, until its pale, thick and significantly increased in volume. I prefer to use an electric whisk for this.
3. In a separate bowl sift together the plain flour and baking powder and stir in the lemon zest.
4. Gently but swiftly mix the dry ingredients into the egg mixture.
5. Pass the cool melted butter through a sieve and gently pour it around the sides of the bowl, mixing as you go, trying not to knock any air out of the batter.
6. Cover the bowl and pop in the fridge to chill. In the meantime grease the madeleine moulds well with butter and pop in the fridge to get ice cold. Preheat the oven to 180°C/350°F/gas 4.
7. Once the batter and moulds have chilled down, remove from the fridge and fill the moulds two-thirds full, being careful not to knock out any air. Don't worry if it doesn't fill the mould evenly as it will melt in the oven. Pop the rest of the batter back in the fridge whilst you bake the first batch.
8. Spritz the oven with water and bake the madeleines for 12 - 15 minutes (10 minutes for small ones) until the tops have formed their traditional domes, and are light golden brown.
9. Transfer to a wire rack as soon as they come out of the oven. Wash and grease the mould again and pop in the fridge for 10 minutes, before filling with batter and baking. Repeat until all the batter has been used up.
10. Once the madeleines are cool, dust with icing sugar and serve immediately.
Anjali's Tip: Madeleines are traditionally flavoured and not filled, but you can mix it up any way you like. Sometimes I fill mine with lemon curd to give it an extra lemony flavour. They are best eaten on the day they are baked, but you can keep them in an airtight container for a few days, slightly warming them before serving to keep their light texture.