Preheat oven to 350˚F. Butter a deep 9-inch dish or oven-proof 9×13″ baking dish.
In a large bowl, whisk together the yolks, milk, cream, vanilla, sugar and salt. Tear up the croissants into large pieces (about 1 1/2″ chunks) and add to the bowl. Toss so all the croissant pieces are coated in the liquid. Leave to stand for 15 minutes.
Toss the bowl contents again and add in the blueberries. Transfer to your baking dish and cover with foil. Bake in your preheated oven for 20 minutes. After 20 minutes, remove the foil cover and sprinkle the sugar all over the top. Return to the oven, uncovered, to bake for another 10 minutes or until golden on top. Best enjoyed while warm.