Put the flours, sugar and salt in the bowl of a food mixer fitted with the dough hook and mix together 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 medium and mix until a rough dough is formed. Then continue to knead for a further 10-15 minutes until the dough is smooth and elastic.
With the mixer still on, add the butter a couple of pieces at a time, mixing until fully combined. Once all of the butter has been incorporated, knead the dough at a medium-low speed for 10-15 minutes until the dough no longer sticks to the side of the bowl.
Put the dough in a large lightly oiled bowl and cover with clingfilm. Place the bowl in the fridge and leave it to rise for 8-10 hours.
Next make the creme patissiere so it is cold when you come to use it, put the vanilla extract and milk in a saucepan and bring to the boil.
Meanwhile put the egg and yolks in a bowl and add the sugar and cornflour, whisk until pale and smooth.
Add the boiling milk to the egg mixture and whisk to combine, pour the mixture back into the saucepan and return to a medium heat, whisking until thickened. Cook for a few minutes extra to remove any taste of cornflour.
Pour the custard into a clean bowl, press a piece of clingfilm onto the surface of the creme patissiere and place it in the fridge until needed.
When the dough is ready, grease 8 x 10cm tart tins. Remove the dough from the fridge and press gently to knock it back. Divide the dough into 8 individual pieces and form into rounds. Roll or press the round so there are about 11-12cm in diameter. Put the rounds of dough into the prepared tins. Allow the excess to go up the sides of the tin as if lining it with pastry. This creates a depression in the middle which will later be filled with creme patissiere and blueberries. Cover the rolls with a sheet of lightly oiled clingfilm and leave in a warm place to prove for about two hours until almost doubled in size.
Preheat the oven to 180C/160C Fan/Gas 4. To test the dough has proved fully, press it 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.
Lightly whisk the remaining egg with a teaspoon of water and brush the ridge of the dough, then sprinkle over a little pearl sugar. Fill the depression with a layer of creme patissiere and top with the blueberries.
Bake for 20-25mins until the bread is golden brown in colour. Remove from the oven and leave to cool in the tins for 10 minutes, then carefully turn out onto a wire rack to cool completely.
These buns are best on the day they are made, but they can also be frozen for up to one month.