Madeleines are a lovely shell-shaped sponge cake. A properly made madeleine has a little bump in the centre of it, called “la bosse” and there are a few easy techniques to achieve this.
Whip the eggs with the sugar, citrus zest and vanilla with electric beaters or in a stand mixer fitted with the whip attachment on high speed until the eggs are thick and pale and hold a ribbon when the beaters are lifted, about 3 minutes.
Sift the flour and baking powder together and add this to the eggs while beating on medium-low speed, mixing until blended. In a separate bowl, whisk the butter, milk and oil together. Spoon about a cup of the batter into the butter and whisk it in. Add this all back to the base batter and fold until blended. Cover and chill the batter for 2 hours, to set the butter.
Preheat the oven to 375 °F (190 °C). Lightly grease and flour 2 madeleine pans , tapping out any excess flour.
Stir the chilled madeleine batter gently to deflate it slightly (this is key to help getting the little bump that a well-made madeleine has) and with an ice cream scoop or a piping bag fitted with a plain tip, fill the madeleine molds (they will fill ¾ full). Bake the madeleines for 10 minutes, then bang the pans down (again, for the bump) before returning to the oven for another 5-7 minutes, until the madeleines are golden brown at the edges. Remove the madeleines from the pan immediately and enjoy warm or at room temperature.
Madeleines are best enjoyed the day they are made.