Preheat the waffle baker and grease it with cooking spray.
In large bowl, whisk together oil, buttermilk and eggs. Stir in the mashed potatoes, scallions and cheese until well combined.
In a separate small bowl, whisk together the flour, baking powder and baking soda. Fold the flour mixture into the potato mixture until it's well combined.
Scoop 1/2 to 2/3 cup (depending on the size of your waffle baker) of the mixture into the prepared waffle baker, spreading it into an even layer. (The potato mixture will not spread or expand as much as a regular waffle, so it's important to spread it in an even layer.) Close the lid and let the waffle bake until golden brown and the egg is cooked throughout.
Transfer the waffle to a serving plate then repeat the filling and baking process with the remaining potato mixture.
Serve the potato waffles topped with sour cream and garnished with additional chopped scallions.