Butter a 9x13-inch dish; spread bread evenly in it.
Whisk eggs, milk, cream, sugar, vanilla, cinnamon, nutmeg, and salt in a large bowl until smooth and combined.
Pour evenly over bread; press gently to soak all pieces and top with optional blueberries.
Drizzle butter over entire dish; cover with plastic wrap and refrigerate at least 30 minutes, preferably overnight.
Preheat oven to 350°F; remove plastic wrap and stand dish at room temperature while oven heats.
Bake uncovered 45-50 minutes until top is golden brown and center sets with a slight jiggle when shaken.
Cool 10 minutes; dust with powdered sugar and drizzle with warm maple syrup. Serve with whipped cream or vanilla yogurt if desired.