Preheat your Belgian waffle maker to medium-high heat. In a large bowl, whisk together the all-purpose flour, cornstarch, brown sugar, baking powder, baking soda, salt, and pumpkin pie spice until evenly blended.

Separate the eggs, placing the whites in a medium glass or metal bowl and the yolks in another bowl. Whisk the pumpkin puree, buttermilk, melted butter, egg yolks, and vanilla extract into the yolk bowl until smooth.

Using a hand mixer or whisk, beat egg whites until stiff peaks form (about 2 minutes). Pour the pumpkin-buttermilk mixture into the dry flour bowl and stir gently until just combined (a few small lumps are fine). Gently fold in the whipped egg whites in two batches using a rubber spatula until airy and uniform.

Lightly brush or spray the hot waffle grids with neutral oil. Pour about 1/2 to 2/3 cup of batter (depending on your waffle maker size) onto the center. Close and bake for 4 to 5 minutes until steam stops escaping and the exterior is deeply golden-brown and audibly crispy.

Beat softened butter, 3 tbsp maple syrup, and cinnamon together with a fork until fluffy. Stack hot waffles high, crown with a generous scoop of whipped maple butter, drizzle warm pure maple syrup into every deep pocket, scatter toasted pecans over top, and serve immediately!