Pour approximately 2 tablespoons of oil into a cast-iron skillet and heat over medium heat.
Scoop out heaping tablespoons of mixture into the hot oil. Dip the back of a spoon into the oil and then the mashed potato mixture to flatten to about a 1/4-inch thick. By dipping the back of the spoon into the oil, the spoon will not stick to the potatoes, making flattening them much easier.
Fry until golden brown and then flip and continue to brown the potato patty on the other side.
Remove to a paper towel-lined plate to serve.
Note from Mama: I do not use onions in my potato cake recipe since my grandmother never did. It is strictly personal preference. But if you do, make sure to dice them finely so that they will cook in the short amount of time it takes for patties to brown.