Grease a 9x13 inch baking pan well. Lay out bread slices in two rows, overlapping. In a large bowl, whisk together the eggs, milk, cream, sugars, and vanilla until well combined. Pour over bread. Cover with plastic wrap and place in fridge overnight.
Preheat oven to 350 degrees. Make the topping but whisking together the flour, sugar, cinnamon, and salt. Add diced or grated butter (see * note for clarity) and toss in the flour mixture. Roll the butter and flour through your fingers to form sand-like consistency, some larger pieces are fine.
Sprinkle the pecans over the custard soaked bread. Sprinkle the topping evenly on top, it seems like a lot, but use it all. Bake for 45 minutes for a custard-like texture or 60 minutes for a crisper texture. Let cool slightly, serve with maple syrup or powdered sugar, or both!
* The butter needs to be super cold. You can either small dice it or do what I do, and grate it right into the bowl with the flour mixture.