Grease a 9x13-inch ceramic baking dish generously with softened butter. Arrange the cubed brioche bread evenly throughout the prepared dish in a loose, fluffy layer.

In a large bowl, vigorously whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla extract, cinnamon, nutmeg, and sea salt until completely smooth and emulsified with no visible egg streaks.

Slowly pour the rich custard mixture evenly over the cubed brioche, pressing down gently with your hands or a spatula so all bread cubes absorb the liquid. Cover tightly with foil and refrigerate for at least 4 hours, or ideally overnight, allowing the buttery bread to transform into a decadent bread pudding texture.

Preheat oven to 350°F (175°C). In a small bowl, prepare the streusel by combining flour, brown sugar, and cinnamon. Cut in the cold cubed butter using a pastry blender or your fingertips until crumbly, then fold in the chopped pecans. Scatter the streusel evenly over the soaked casserole.

Bake covered with foil for 25 minutes. Remove the foil and bake uncovered for another 20 to 25 minutes until puffed, golden-brown, and a knife inserted into the center emerges clean without wet custard. Dust with powdered sugar and serve warm with pure maple syrup!