In a small bowl, stir together the pumpkin puree, chili powder, sage, and cayenne until fully combined. I like to taste a tiny bit at this stage to check the spice level. If you want it milder, cut the cayenne in half, and if you like more heat, feel free to add a touch extra.
Heat your griddle or large skillet over medium heat and lightly coat it with nonstick spray or a thin layer of butter. Give it a minute or two to heat evenly before adding any tortillas, since a cold pan leads to soggy quesadillas instead of that crisp golden crust.
Lay four tortillas flat on your work surface and spread about a quarter of the pumpkin mixture over each one in a thin, even layer. Don't go overboard here, a thin layer melts evenly and keeps the quesadilla from getting soggy in the middle.
Layer the thinly sliced brie over the pumpkin mixture, then top each with a second tortilla to close it up. I try to spread the brie slices out so every bite gets that gooey cheese pull.
Place the assembled quesadillas on the hot griddle and cook for about 2 to 3 minutes per side, pressing gently with your spatula. You're looking for a golden-brown tortilla and cheese that's melted all the way through.
Transfer the quesadillas to a cutting board and let them rest for a minute before slicing into wedges. This short rest keeps the melted brie from oozing out everywhere the second you cut in.