Place potato chunks in a large pot of salted water and bring to a boil. Cook for 15 to 20 minutes until fork-tender.
Drain potatoes and return to the pot. Mash with 2 tablespoons of butter, milk, and salt until smooth and creamy.
Pat steak cubes dry with paper towels and season generously with salt and black pepper.
Heat olive oil in a large cast-iron skillet over high heat until shimmering.
Add steak cubes in a single layer, working in batches if necessary, and sear for 2 minutes without moving to develop a crust.
Turn steak cubes and cook for an additional 2 minutes until browned on all sides.
Reduce heat to medium-low. Add the remaining 4 tablespoons of butter and minced garlic to the skillet.
Toss the steak cubes in the melting garlic butter for 1 minute until fragrant and well-coated.
Remove from heat and stir in fresh parsley.
Serve the garlic butter steak bites immediately over a bed of the warm mashed potatoes.