For the beef: Heat the olive oil in a cast-iron skillet over medium-high heat. Sprinkle the sirloin cubes with 1 teaspoon salt and 1/2 teaspoon pepper. In batches if necessary, sear the meat on all sides and cook until medium rare, 2 to 3 minutes. Drain on paper towels and set aside to cool.
Meanwhile, make the dipping sauce: In a bowl, whisk together the mayonnaise, lime zest and juice, vinegar, sesame oil, sugar, red pepper flakes and some salt and pepper.
Thread the beef onto toothpicks (2 pieces per). Garnish with cilantro and serve with the dipping sauce.