Preheat your grill or stovetop grill pan over medium-high heat.
In a small bowl, combine the softened butter with Worcestershire sauce, Dijon mustard, parsley, garlic, lemon juice, and cayenne pepper (if using). Mix until well combined and set aside.
Rub the flank steak with olive oil, salt, black pepper, garlic powder, onion powder, and smoked paprika.
Grill the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from the grill and let it rest for 5 minutes.
While the steak is resting, slice the slider buns in half and toast them lightly on the grill or in a toaster.
Slice the steak against the grain into thin strips.
Spread a generous amount of cowboy butter on the bottom half of each slider bun. Layer with shredded lettuce, sliced tomatoes, and steak strips. Top with the other half of the bun.
Serve immediately and enjoy!