Generously grease a 9x5-inch loaf pan with the softened butter. Grease everywhere, but focus more on the sides and corners than the bottom.
Prepare the topping: In a small bowl, combine the melted butter and brown sugar. Pour into the loaf pan, spreading evenly with a spatula. Top evenly with the pecans, gently pressing them into the mixture.
Add the bread to the loaf pan, gently packing it down as you add more.
In a large bowl, whisk together the eggs, milk, maple syrup, vanilla, and brown sugar, if using. Slowly pour the mixture into the loaf pan, periodically pressing the bread down as it absorbs the liquid.
Cover with plastic wrap and refrigerate for at least 30 minutes or overnight.
Preheat the oven to 350 degrees F. Remove the plastic wrap, and place the loaf pan on a baking sheet lined with parchment or foil (this will make cleanup easier if the topping bubbles over). Bake for 60 minutes.
Allow to cool for 5-10 minutes, then run a knife or offset spatula around the edges of the pan. Carefully flip the loaf pan onto a serving plate and lift up. Spoon any topping that remains in the pan over the French toast. Serve immediately.
Adding dark brown sugar to the filling will create a sweet (but not cloying) French toast. French toast isn’t traditionally sweet since it’s served with syrup, and you likely won’t need to serve this with anything. If you’d prefer that only the topping is sweet so that this can be served with maple syrup, omit the 1/4 cup brown sugar from the filling for a more neutral flavor.