Begin by making the custard. Place the cashew milk and vanilla in a small saucepan over medium heat and bring just to the boil. Place the egg yolks, sugar and cornflour in a heatproof bowl and whisk to combine. Gradually whisk in the milk mixture until combined. Return to the saucepan and place over low heat and whisk for 3 to 5 minutes or until thickened. The mixture should coat the back of a wooden spoon and leave a slight trail when you remove the spoon from the pan and let the custard drop back into the pan. Place into a heatproof bowl and cover with plastic wrap and refrigerate until cool and spreadable.
While the custard is cooling make the brioche. Place the yeast and milk in a small bowl or cup and stir to dissolve yeast. Leave for 5 minutes or until foamy. Add the flour, sugar, eggs, yeast mixture and sea salt to the bowl of a stand mixer fitted with a paddle attachment and mix until the dough comes together. Switch the paddle attachment for a dough hook and knead, gradually adding the olive oil, a tsp at a time, allowing the oil to become fully incorporated before adding more. For a little bit it will look like a hot oily mess, but stick with it, the oil will mix into the dough with a little patience. Knead for 10 minutes or until the dough smooth and elastic. Cover dough with a tea towel and leave in a warm place for 11/2 to 2 hours or until doubled in size.
Knock back the dough and place on a tray covered with baking paper that has been lightly floured. Press down and use your fingers to shape into a rough circle. Lightly brush the top with more oil and cover with a tea towel and leave an hour or until doubled in size. While the dough is rising preheat the oven to 220 celsius (425 Fahrenheit).
When the brioche is ready to be baked spread half the custard over the top of the base, making sure not to go all the way to the edge. Cut into whatever shapes you like, just be sure that the rhubarb is not too thick otherwise it won't cook in time. Toss the rhubarb in the extra sugar and arrange on the brioche base in whatever pattern you desire. Bake for 12 to 18 minutes or until golden. Check after 10 minutes to make sure it isn't browning too much.
Serve dusted with icing sugar and extra custard.
Feel free to swap the cashew milk with your preferred milk.