For the French toast: Grease the baking pan with butter. Cut bread into cubes, and evenly distribute in the pan. Crack the eggs in a big bowl. Whisk together the eggs, milk, cream, granulated sugar, brown sugar and vanilla. Pour evenly over the bread (can be covered and stored overnight).
For the topping: Mix the flour, brown sugar, cinnamon, and salt in a separate bowl. Stir together using a fork. Cut butter in to smaller pieces and add the butter, and mix it all together until the mixture resembles fine pebbles.
When you're ready to bake the casserole, preheat the oven to 350 degrees F. Sprinkle the topping over the top. Bake for 45 to 1 Hour+ depending on desired firmness.
Scoop out individual portions. Drizzle with pancake syrup.