Mix the flour, ground almonds, chocolate powder, sugar, and salt in a bowl.
Add the butter and rub together by hand to form an even, crumbly mixture.
Add the egg white and mix quickly to form a soft dough; do not knead.
Flatten the dough, cover, and chill for about 30 minutes.
Roll out the dough into a circle about 28 cm in diameter between two sheets of baking paper.
Transfer to a 24 cm flan tin with the bottom sheet of baking paper and prick the base firmly with a fork.
To Bake
Bake for about 20 minutes in the lower half of an oven preheated to 200°C.
Remove from the oven, allow to cool slightly, then leave the tart base to cool completely on a rack.
Lime And Basil Creme
In a pan, whisk the water with the sugar, butter, cornflour, eggs, and salt.
Bring to the boil over medium heat, stirring continuously.
As soon as the mixture thickens, remove the pan from the heat, continue whisking for about 2 minutes, then strain through a sieve and allow to cool slightly.
Puree the basil with about 100 ml lime juice, mix into the creme, and pour over the tart base.
Leave to cool, then cover the tart and chill for at least 4 hours.
Decoration
Decorate the tart with lime slices and basil leaves.