Pat the sirloin steak cubes dry with paper towels. Season evenly with kosher salt and freshly ground black pepper.
Heat olive oil in a large skillet over medium-high heat until shimmering.
Add the steak bites to the skillet in a single layer, making sure not to overcrowd the pan. Sear for 2-3 minutes without stirring to develop a crust.
Flip the steak bites and cook for an additional 2-3 minutes until browned and cooked to desired doneness. Remove steak bites from the skillet and set aside on a plate.
Reduce heat to medium and add 4 tablespoons of unsalted butter to the same skillet.
Once the butter has melted, add the minced garlic, chopped parsley, fresh thyme leaves, smoked paprika, and cayenne pepper. Stir and cook for 1-2 minutes until fragrant.
Add the remaining 2 tablespoons of butter and fresh lemon juice to the skillet. Stir until the butter is melted and the sauce is well combined.
Return the steak bites to the skillet and toss to coat evenly in the cowboy butter sauce.
Cook for an additional 1-2 minutes to heat through and allow the flavors to meld.
Remove from heat and serve immediately.