Put the flours, sugar and salt in the bowl of your mixer fitted with the dough hook and mix to combine. Put the milk and yeast in a jug and mix together until the yeast has dissolved. Pour the mixture into the mixer followed by the three eggs. Turn the mixer on to low and mix until a rough dough is formed. Knead for 10 minutes using the mixer until the dough is smooth and elastic.
With the mixer still on, add the butter, a couple of pieces at a time, mixing until full combined. Once all the butter has been incorporated, knead the dough at a low speed for 10-15 mins until the dough no longer sticks to the side of the bowl. Put the dough in a large, lightly greased bowl and cover with clingfilm. Transfer to the fridge and leave to rise slowly for 8-10 hours.
Next morning line two baking trays with baking parchment. Remove the dough from the fridge and tip it out onto a lightly floured work surface. Diving into eight equal pieces and form into rounds. Flattening each into a disc. Put the rounds onto the baking trays and cover with lightly greased clingfilm. Put the trays in a warm place and leave to prove for about 2 hours until risen and puffy. To test the dough has proved fully, press with a lightly floured finger. If the dough springs back slowly, it is ready for baking, if it springs back quickly, it needs a little more time to prove.
While the brioche is proving, make the creme patissiere, take the milk and vanilla extract, place them in a pan and gently bring the milk to a boil.
In a separate bowl combine the egg, egg yolks, caster sugar and cornflour in a bowl. Whisk until pale and creamy. Add the milk to the eggs and mix until thoroughly combined, return the mixture to the pan and stir until thickened and all of the flour is cooked out of the creme patissiere.
Place the creme patissiere in the bowl, cover it with clingfilm, making sure the clingfilm touches the top of the cream. Place the cream in the fridge to cool.
Preheat the oven to 180C/160C fan/gas 4, if using an AEG SteamBake oven add your 100ml of water and choose the SteamBake option and press the SteamBake button and wait for the oven to preheat.
Lightly whisk the remaining egg and brush the buns to coat. Sprinkle the buns with pearl sugar and then bake for 15-20mins until risen and golden brown. Remove from the oven and leave to cool on the baking trays for 5 mins, then transfer to a wire rack to cool completely.
When the creme patissiere is cool, whip the whipping cream and mix it into the creme patissiere to lighten it.
Using a serrated knife, slice the tops off the brioche, pipe a ring of creme legere on the base of the brioche and add a dollop of caramel in the middle. Then put the tops back on and gently press them down.
As with all brioche recipes, these are at their best the day they are made, however when left unfilled the buns can be frozen for up to one month.