Preheat the oven to 375°F (190°C). Spray a 9x13-inch (23x33 cm) baking dish with cooking spray.
cooking spray
Cut the brioche into ¾-inch (2 cm) slices. Spread Nutella on one side of each slice, then cut each slice diagonally into two triangles.
1 lb (454 g) brioche , ¾ cup (225 g) Nutella
In a large bowl, whisk together the eggs, heavy cream, vanilla extract, salt, cinnamon, and light brown sugar until smooth.
8 large eggs, 3 cups (720 ml) heavy cream, 2 tsps (10 ml) vanilla extract, ½ tsp (3 g) table salt, ¾ tsp (2 g) ground cinnamon, ⅔ cup (145 g) packed light brown sugar
Pour half of the custard into the prepared baking dish. Arrange the Nutella-coated bread triangles, Nutella side up, in an overlapping zigzag pattern. Pour the remaining custard evenly over the top and gently press the bread down to help it absorb the mixture. Cover and refrigerate for at least 2 hours or up to 24 hours.
Remove from the refrigerator while the oven preheats.
Bake uncovered for 20 minutes. Loosely cover with foil and bake an additional 15 to 20 minutes, until the custard is set and the center no longer appears liquid.
Meanwhile, combine brown sugar, water, and cinnamon in a small saucepan over medium heat. Whisk until the sugar dissolves and the mixture comes to a boil. Remove from heat immediately; it should be thin and syrupy.
⅓ cup (70 g) packed light brown sugar, 3 tbsps (45 ml) water, ¼ tsp (0.5 g) ground cinnamon
Brush the syrup evenly over the baked French toast. Return to the oven and bake uncovered for 5 to 10 minutes more, until the top is shiny and lightly crackled.
Remove from the oven and rest for 5 to 10 minutes before serving. Dust with confectioners' sugar if desired.
confectioners' sugar for dustng