In a medium bowl, combine the soy sauce, minced garlic, brown sugar, black pepper, and red pepper flakes. Mix well to create the marinade.
Add the steak cubes to the marinade, cover, and refrigerate for at least 30 minutes to allow the flavors to meld.
Heat a large skillet over medium-high heat and melt 2 tablespoons of butter.
Remove the steak from the marinade (reserve 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 butter and the shrimp. Cook for 2-3 minutes until the shrimp turn pink and opaque.
Pour the reserved marinade into the skillet with the shrimp and bring to a simmer. Cook for an additional 2-3 minutes until the sauce thickens slightly.
Return the steak bites to the skillet and toss everything together to coat in the garlic butter soy glaze.
Serve immediately, garnished with chopped green onions.