Preheat oven to 250°F and preheat waffle iron. Sift together flour, brown sugar, baking powder and soda, salt, and spices. Whisk egg yolks in a large bowl with buttermilk, pumpkin, and butter until smooth. Whisk in dry ingredients just until combined.
In a mixing bowl with a whisk attachment, whisk the egg whites until they hold soft peaks (as in, far softer than the over-beaten whites you’ll see in my picture above). Folk them gently into the waffle batter, until just combined.
Brush waffle iron lightly with oil and spoon batter (about 2 cups for four 4-inch Belgian waffles) into waffle iron, spreading quickly. Cook according to manufacturer’s instructions.
Transfer waffles to rack in oven to keep warm and crisp. Make more waffles in same manner.