Pat the sirloin steak cubes dry with paper towels to remove excess moisture.
Season the steak cubes evenly with kosher salt and freshly ground black pepper.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Add the steak cubes to the skillet in a single layer, making sure not to overcrowd the pan. Cook for 2-3 minutes without stirring to develop a nice sear.
Flip the steak cubes and cook for another 2-3 minutes until browned on all sides and cooked to your desired doneness.
Reduce the heat to medium-low and add the unsalted butter to the skillet.
Once the butter has melted, add the minced garlic and cook for 1 minute, stirring frequently to prevent burning and to infuse the butter with garlic flavor.
Toss the steak bites in the garlic butter until well coated and heated through, about 1-2 minutes.
Remove the skillet from heat and sprinkle the chopped fresh parsley over the steak bites.
Serve immediately, spooning any remaining garlic butter from the pan over the steak bites.