In a large bowl, combine all ingredients (except for butter) and stir until well combined.
Heat the two tablespoons of butter in a large, heavy skillet (such as cast iron) over medium-high heat until melted.
Use a disher scoop (or 1/4 cup measure) to scoop out the potato mixture and add to the skillet. Use the back of a spatula to flatten the patty out so it's about 1/2" thick.
Fry for about 3 minutes, or until golden brown, then flip over and cook for another 3 minutes. Remove to a plate.
Repeat with remaining potato mixture, adding more butter to the skillet if needed.
Serve hot, topped with sour cream and chopped herbs or chives, if desired.
If the potato mixture sticks to the spatula when you flatten it, grease the back of the spatula with some butter or oil, ur use another spatula to help you slide it off.
These mashed potato cakes are best served immediately, when they are crispy, but they are also delicious reheated in the microwave if you have leftovers.
Variations: use leftover roasted veggies, chopped bacon, cheese, jalapeños, or really anything you want in the potato mixture! It's very flexible.
Time saving tip: This recipe cooks up quickly, but you have to cook it in batches since not all the patties fit in one skillet. To save time, do two batches at once in two separate skillets.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.