Scrape out the vanilla pod and bring to the boil together with the cream.
Remove the pod.
Beat the egg yolks, sugar, vanilla sugar and cornflour until fluffy.
Pour the cream over the egg mixture while whisking. Pour back into the pan and bring to boil while whisking constantly. Remove from the heat when the cream has thickened. Pour the cream into a bowl and add the butter a little at a time while whisking. Refrigerate.
Crumble the yeast into a bowl and add the eggs. Mix with a dough hook until the yeast has dissolved.
Add the syrup, salt and flour and knead until soft and smooth. Add the butter a little at a time. Mix for a further 10 minutes or so. Leave the dough to stand covered in the refrigerator for at least 5 hours.
Knead the dough on a floured surface. Roll out thinly, to about 1 cm, and punch out small rounds about 10 cm in diameter. Leave to rise on a baking sheet lined with baking parchment for 1 hour.
Using two fingers, press the dough in the middle to form a small hollow. Brush with beaten egg and pipe the vanilla cream into the hollow. Bake at 220 °C for about 5 minutes.
Leave the brioche to cool. Decorate with fresh berries and sprinkle with icing sugar.
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