If using frozen or firm rice cakes, soak them in warm water for 10 minutes, then drain well.
Heat a pan over medium heat and melt the butter.
Add minced garlic (if using) and sauté for about 20 seconds until fragrant.
Stir in soy sauce and brown sugar (or honey). Add water if needed to loosen the sauce.
Add the rice cakes and toss to coat evenly in the sauce.
Cook for 4–6 minutes, stirring often, until rice cakes are soft and chewy and the sauce thickens.
Turn off heat and drizzle sesame oil (if using).
Top with sesame seeds and green onions, then serve warm.