In a mixing bowl, combine the mashed potatoes, flour, beaten egg, baking powder, salt, and black pepper. Mix until well combined.

Fold in the chopped green onions if using.

Heat butter in a frying pan over medium heat.

Scoop about 1/4 cup of the potato mixture and form it into a patty. Place it in the hot pan.

Cook for about 4-5 minutes on each side, or until golden brown and crispy.

Repeat with the remaining mixture, adding more butter as necessary.

Serve warm, optionally with sour cream or applesauce.