Place halved baby potatoes into a medium pot of cold salted water. Bring to a gentle boil over medium-high heat and simmer for 7 to 8 minutes until just fork-tender. Drain thoroughly and let steam-dry on clean paper towels for 3 minutes.

Pat the sirloin cubes completely dry with paper towels to ensure a crisp sear. Toss in a bowl with 1 tablespoon oil, 1 teaspoon kosher salt, 1/2 teaspoon black pepper, and 1 teaspoon garlic powder until evenly coated.

Heat 1 tablespoon butter and 1 tablespoon oil in a large 12-inch cast-iron skillet over medium-high heat. Place the potatoes cut-side down; sear undisturbed for 4 to 5 minutes until deep golden brown and crispy. Flip and cook for 1 minute more, then transfer to a warm plate.

Turn the heat to high until the skillet is lightly smoking. Add the steak bites in a single layer with space between cubes (sear in two batches if necessary). Sear undisturbed for 2 minutes to create a mahogany caramelized crust, then flip with tongs and cook for another 1 to 2 minutes for medium-rare to medium tenderness.

Reduce heat to low. Return the crispy golden potatoes to the skillet alongside the steak bites. Add the remaining 3 tablespoons butter, minced garlic, fresh thyme, and chopped rosemary.

As the butter foams, spoon it continuously over the steak bites and potatoes for 60 seconds until fragrant. Remove immediately from heat. Garnish with chopped flat-leaf parsley and flaky sea salt. Serve sizzling hot directly from the skillet.