Grease a 9x13-inch baking dish with the softened butter.
Spread the bread cubes in the dish in an even layer.
Whisk eggs, milk, half-and-half, sugar, vanilla, cinnamon, nutmeg, and salt until smooth.
Pour the custard over the bread. Press gently so every piece absorbs liquid.
Cover tightly and refrigerate overnight, at least 8 hours.
Stir flour, brown sugar, cinnamon, and a pinch of salt in a bowl.
Drizzle in melted butter and mix until crumbles form. Fold in pecans.
Uncover the casserole. Let stand 15 minutes while heating oven to 350°F.
Sprinkle streusel evenly over the top. Bake 40–50 minutes until puffed and set.
Rest 10 minutes before serving. Drizzle with warm maple syrup.