How to Make It
Preheat oven to 400°F. Place waffles on two baking sheets. (Note: some of the waffles can overlap slightly). Bake until waffles are warmed through and lightly toasted, about 10 minutes. Remove and set aside.
In a mixer fitted with the whisk attachment on medium-high, beat together eggs, sugar, and 1/4 cup maple syrup until tripled in volume, about 5 minutes. Add bourbon, vanilla, and salt; mix to combine. On low speed, stream in cream and milk until fully incorporated.
Grease a 9-x-13-inch baking dish with cooking spray. Cut each of the waffles in half. Place waffle halves in baking dish, layering them about 3 layers deep. Pour egg mixture into baking dish, using a large spoon to press down on the waffles to encourage the custard to soak into the waffles. Cover with aluminum foil and refrigerate for 8 hours or overnight.
Preheat oven to 325°F. Bake waffle casserole with aluminum foil for 1 hour. Remove aluminum foil and bake until the surface is lightly browned and the custard is set, about 20 minutes.
Slice casserole and serve with butter and additional maple syrup.