Grease or line a 26 cm round pan with parchment paper.
Place whole croissants in the pan. If using larger ones or bread, slice them into chunks.
Whisk eggs, milk, cream, sugar, vanilla, and salt in a large bowl until smooth.
Pour custard over the croissants and press down gently to soak.
Place a plate on top to keep croissants submerged. Soak in the fridge for 30 minutes to 1.5 hours.
Preheat the oven to 170°C (340°F) while soaking.
Sprinkle blueberries on top. For a crispy crust, add powdered sugar before baking.
Bake for 45-55 minutes, checking after 35 minutes. If the top starts browning too quickly, loosely cover with parchment paper. Continue baking until the pudding is golden and risen.
Make the Vanilla Sauce: For the sauce, combine cream, milk, cornstarch, and sugar in a small pot, whisking well to dissolve the cornstarch. Cook over low heat, stirring for 5-10 minutes until the sauce thickens. Remove from heat and stir in vanilla and butter until smooth.
Once baked, let the bread pudding rest for 15 minutes to set before serving.