Boil 4 cups (1/2-inch diced) russet potatoes just until fork tender, 5-10 minutes. Drain well.
In a large skillet over medium heat, add 4 Tablespoons of unsalted butter. Once melted, add the cooked potatoes, 1 small sweet onion, small diced, 1/2 teaspoon kosher salt and 1/2 teaspoon black pepper. Cook, occasionally stirring, until the onions are softened and the potatoes and onion start to brown, 8-10 minutes.
Add 4 cups chopped cooked corned beef, 1/2 teaspoon garlic powder and 1/2 teaspoon onion powder to the pan. Stir, so everything is combined.
Taking the back of a spatula, press the mixture down to form an even layer. Cook for 3-5 minutes or until a golden crust begins to form on the bottom.
Scrape the bottom of the pan and flip up the hash, so the crusty part is on top. Add the remaining 2 Tablespoons unsalted butter, and mix it in.
Again, using the spatula, press the hash down to form an even layer. Cook another 3-5 minutes until golden on the bottom. Continue doing this until you reach your desired browning. I usually do this 3 times. Then serve!