Lightly butter a 9-by-13 baking dish, then place cubes of brioche in the baking dish in an even layer.
In a large bowl, whisk together eggs, milk, heavy cream, sugars, vanilla, bourbon and a pinch of sea salt. Sprinkle toasted pecans over the brioche, then pour the custard evenly all over the bread. Gently press the bread down into the liquid. Cover and let sit in the fridge for at least 2 hours (or up to overnight).
Once you are ready to bake the bread pudding, take the baking dish out of the fridge and preheat an oven to 350°F. Meanwhile, prepare the crumble. Add flour, pecans dark brown sugar, cinnamon, salt and butter to a medium bowl, and using your fingertips, rub ingredients together until evenly combined and clumps start to form. Sprinkle the surface of the bread evenly with the crumbs. Bake until the bread pudding is puffy and the top is golden (45 minutes to an hour). Let cool for a few moments before serving.
To serve, cut squares of warm bread pudding, and place on small plates. Top with sliced persimmons and maple syrup. Enjoy immediately!