Preheat the oven to 180°C.
Line a 6 cup shallow cupcake tin with 6 cupcake cases.
Combine the eggs, vanilla extract and sugar in a large heatproof mixing bowl. Sit over a pan of gently simmering water and whisk until the mixture is thick and pale and leaves a trail when the whisks are lifted out. Remove from the heat and fold in the flour until incorporated.
Spoon the batter into the cupcake cases and bake for 15-20 minutes until golden and risen. Remove and allow to cool.
Meanwhile, prepare the custard by bringing the milk, cream and vanilla extract to the boil in a medium saucepan over a medium heat. Whisk together the egg yolks, cornflour and sugar in a mixing bowl until pale and thick. Pour the hot cream and milk over the top, whisking simultaneously until you have an even mixture. Transfer back to the saucepan and cook over a low heat, stirring constantly until thickened. Remove and cool.
Cut little ravines out of the centre of the sponges, leaving the texture uneven and making sure there are pockets for the custard to be poured into.
Once the custard has cooled, whisk again until smooth and pour into the centre, allowing it the settle naturally in the contours. Chill until set, then serve.