In a medium bowl, whisk together the soy sauce, brown sugar, sesame oil, rice vinegar, garlic, ginger, black pepper, and red pepper flakes (if using).
Add the thinly sliced beef to the marinade, making sure all the pieces are well coated. Cover the bowl and refrigerate for at least 30 minutes, or up to 2 hours.
Heat a large skillet or griddle over medium-high heat. Once hot, add the marinated beef in a single layer. You might need to do this in batches.
Cook for about 2-3 minutes per side, or until the beef is cooked through and slightly caramelized.
While the beef is cooking, slice the mini croissants in half horizontally.
Once the beef is cooked, remove it from the pan and let it rest for a few minutes. Then, fill each croissant with a generous portion of the Korean BBQ beef.
Top with shredded carrots and chopped green onions. Sprinkle with sesame seeds, if desired.
Serve warm and Enjoy.