Crack the eggs into a wide, shallow dish. Pour in the melted unsalted butter and whisk until the two ingredients are thoroughly incorporated.
Add the whole milk, granulated sugar, vanilla extract, and pumpkin spice mixture. Continue whisking until the sugar is dispersed and the custard has a smooth, uniform consistency.
Arrange the thick pumpkin brioche slices in the prepared custard.
Let each piece absorb the mixture for 30 to 60 seconds on each side. The bread should become well saturated and flavorful without softening to the point of becoming soggy.
Lightly coat a skillet or griddle with cooking spray, then warm it over medium-low heat.
Transfer the soaked brioche slices to the heated cooking surface. Cook each side for 2 to 3 minutes, or until the exterior develops an even golden-brown color and the interior remains soft.
Continue cooking gently at the lower temperature when necessary to create a crisp surface without drying out the center.
Remove the finished French toast from the skillet or griddle and serve immediately while warm.
Drizzle with maple syrup according to preference.