In a mixing bowl containing a little warm water, pour in a sachet of yeast stirring in 50 g of flour little by little and leave aside to ferment until the mixture doubles in size.
In another bowl mix together 150 g sugar, lemon zest, 3 eggs, a pinch of salt, 25 g of softened butter and gradually add 500 g of flour, stirring all the time.
Add the yeast mixture and knead for a few minutes before covering with a wet tea towel and leaving to rest for 3 hours.
Place the dough in a high-sided cake tin dusted with flour, brush with egg yolk and bake at 200°C for half an hour.
When cooked, turn out and leave to cool, then slice in half through the middle.
Prepare the custard filling by vigorously whisking together 3 eggs, 5 spoonfuls of sugar+ 2 of flour, 1/2 litre of boiling milk, a drop of vanilla essence and pour the mixture into a preheated saucepan.
Whip 250 g of fresh cream and gently fold in the custard mixture.
Spread over one of the brioche layers and sandwich together.