In a medium bowl, combine the soy sauce, minced garlic, brown sugar, black pepper, and red pepper flakes (if using). Mix well to create the marinade.

Add the steak cubes to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.

Heat a large skillet over medium-high heat. Add 1 tablespoon of butter and let it melt.

Remove the steak from the marinade and add it to the skillet. Sear the steak bites for about 3-4 minutes, turning occasionally, until browned and cooked to your desired doneness. Remove the steak from the skillet and set aside.

In the same skillet, add the remaining tablespoon of butter. Once melted, add the shrimp and cook for 2-3 minutes, or until they turn pink and opaque.

Return the steak to the skillet with the shrimp. Pour the remaining marinade over the mixture and cook for an additional 1-2 minutes, allowing the sauce to thicken slightly.

Remove from heat and garnish with chopped green onions before serving.