Melt butter in a small saucepan over medium-low heat. Add chopped pecans then cook, stirring occasionally, until pecans are light golden brown, 8-10 minutes. Add maple syrup then stir to combine and turn off the heat. Let syrup sit and infuse while preparing the rest of the casserole.
For the French Toast Casserole:
Spray a 9x13” baking dish with nonstick spray then set aside. Trim away any tough crusts from the bread slices then slice into bite-sized cubes and arrange in the bottom of prepared baking dish.
In a large bowl, whisk together eggs, milk, 1/2 cup Butter-Pecan Maple Syrup, vanilla and cinnamon until smooth then pour over the cubed bread and press down with a spatula to ensure all the bread is soaked. Cover with foil then refrigerate for 2-12 hours. Refrigerate remaining Butter-Pecan Maple Syrup. (I leave it in the saucepan with a lid on or transfer to a mason jar with a lid.)
Preheat oven to 350 degrees. Pull the baking dish out of the refrigerator to warm up a bit then gently press on the bread cubes again with a spatula to evenly distribute the egg mixture - do not pack bread cubes down though. Bake for 40-45 minutes or until the eggs are set in the center and the top is golden brown and slightly crispy. Check the casserole around the 30 minute mark - if the top is browning too quickly, place a piece of foil on top of the baking dish then continue baking.
Let casserole cool for 10 minutes before slicing. Warm remaining Butter-Pecan Maple Syrup then serve with fresh berries, if using.