Combine the flour and the ingredients through the brown sugar in a mixing bowl. In a separate bowl, stir together the pumpkin, milk and egg yolks. Whip the egg whites in a separate bowl until soft peaks form.
Stir the flour mixture and the melted butter into the pumpkin mixture until just combined. Gently fold in the egg whites, about one-third at a time, stirring until incorporated. Cook waffles according to manufacturer's instructions.
For the syrup:
Stir together the sugar, cornstarch, and cinnamon in a saucepan. Add the apple cider. Cook over medium heat until the mixture starts to boil; boil until mixture thickens. Remove from heat and stir in 2 tablespoons butter, if desired.