Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.
In a mixing bowl, toss the halved baby potatoes with olive oil, salt, and black pepper. Spread them out on the prepared baking sheet in a single layer.
Roast the potatoes in the preheated oven for about 20-25 minutes or until they are golden brown and tender, flipping them halfway through.
While the potatoes are roasting, season the cubed steak with salt and pepper.
In a large skillet, melt 2 tablespoons of butter over medium-high heat. Add the seasoned steak bites to the skillet in a single layer. Cook for about 2-3 minutes without stirring to get a nice sear.
Stir the steak bites and continue cooking for another 2-3 minutes until cooked to your desired doneness (medium rare is recommended).
Reduce the heat to low, then add the minced garlic, chopped rosemary, and the remaining 2 tablespoons of butter to the skillet. Cook for another 1-2 minutes until the garlic is fragrant and the butter is melted, stirring to coat the steak bites.
Remove from heat and sprinkle the fresh parsley over the steak bites.
Serve the garlic butter steak bites alongside the roasted potatoes, drizzling any leftover garlic butter sauce over the top.