Place the peeled and chunked potatoes into a large pot and cover with cold water. Add 1 teaspoon kosher salt to the water.
Bring the pot to a boil over high heat. Reduce heat to medium and simmer until potatoes are tender when pierced with a fork, about 15-20 minutes.
While the potatoes cook, pat the steak cubes dry with paper towels to remove excess moisture. Season the steak bites with 1 teaspoon kosher salt and 1/2 teaspoon black pepper evenly.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering.
Add the steak bites in a single layer, working in batches if necessary to avoid overcrowding. Cook without stirring for 2-3 minutes to develop a crust.
Stir and continue cooking for another 2-3 minutes until steak bites are browned and cooked to desired doneness.
Reduce heat to medium-low and add 2 tablespoons unsalted butter and minced garlic to the skillet. Stir to coat the steak bites in garlic butter and cook for 1-2 minutes until fragrant. Remove skillet from heat and keep warm.
Drain the cooked potatoes and return them to the pot. Add the remaining 2 tablespoons unsalted butter, warmed whole milk, and sour cream.
Mash the potatoes with a potato masher or fork until smooth and creamy. Season with 1 teaspoon kosher salt and 1/2 teaspoon black pepper, adjusting to taste.
Divide the mashed potatoes among four plates. Spoon the garlic butter steak bites over or beside the mash.
Garnish with chopped fresh parsley and serve immediately.