Place the peeled and chopped potatoes in 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. Season them evenly with 1 teaspoon kosher salt and 1/2 teaspoon black pepper.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat until shimmering.
Add the steak cubes in a single layer, working in batches if necessary to avoid overcrowding. Cook for 2-3 minutes without stirring to develop a crust.
Flip the steak bites and cook for another 2 minutes until browned and cooked to desired doneness.
Reduce heat to medium-low. Add 2 tablespoons of butter and the minced garlic to the skillet. Stir to coat the steak bites in the garlic butter and cook for 1 more minute. Remove from heat and set aside.
Drain the cooked potatoes and return them to the pot. Add the remaining 1 tablespoon of butter, warmed milk, and sour cream.
Mash the potatoes using a potato masher or hand mixer 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. Top with garlic butter steak bites and spoon any remaining garlic butter from the skillet over the steak.
Garnish with chopped fresh parsley and serve immediately.