Spray a heat proof 8-inch glass bowl with nonstick cooking spray (this is the bowl you will use to cook your French toast in). Set aside.
Make the almond butter cinnamon drizzle: In a small bowl, mix together the almond butter, maple syrup, cinnamon, and melted coconut oil. The consistency should be drippy - if not, add additional melted coconut oil 1 teaspoon at a time.
In a medium bowl, whisk the eggs, milk, vanilla, and maple syrup together.
Assemble the French toast casserole: Place half of the bread cubes and half of the walnuts into the prepared glass bowl. Drizzle half of the almond butter cinnamon drizzle over the bread mixture. Add the rest of the bread cubes and walnuts and drizzle the remaining almond butter mixture on top.
Pour the egg mixture over the bread cubes and use a spatula to mix. If adding the optional topping, sprinkle the cubed butter and brown sugar on top.
Place the metal Instant Pot trivet on the bottom of Instant Pot and pour in 1 cup of water. Place the bowl of the French toast casserole mixture on top of the trivet.
Lock the Instant Pot and turn the pressure valve to seal. Press manual and turn to high pressure and cook for 25 minutes.
Quick release and remove Instant Pot cover. Let cool and serve with your favorite French toast toppings.