Pat the sirloin steak cubes dry with paper towels.
Season the steak cubes 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 cubes to the skillet in a single layer, cooking in batches if necessary to avoid overcrowding.
Sear the steak bites for 2-3 minutes on each side until browned and cooked to desired doneness.
Remove the steak bites from the skillet and set aside on a plate.
In a small bowl, combine the softened butter, minced garlic, chopped parsley, chopped chives, smoked paprika, cayenne pepper, and fresh lemon juice. Mix thoroughly to create the cowboy butter.
Reduce the heat to low and return the skillet to the stove.
Add the cowboy butter mixture to the skillet and melt it gently, stirring to combine with the pan juices.
Return the steak bites to the skillet and toss them in the cowboy butter sauce until evenly coated and heated through, about 1-2 minutes.
Remove from heat and transfer the steak bites to a serving dish.
Serve immediately, garnished with additional chopped parsley if desired.