Preheat your oven to 375°F and line a large baking sheet with parchment paper.
Microwave 6 tbsp salted butter and 1½ tbsp minced garlic together in a small microwave-safe bowl for 20 seconds to mellow out the sharp garlic flavor.
Stir ¼ tsp garlic powder, ¼ tsp kosher salt, 1–2 tbsp fresh parsley, and a pinch of black pepper directly into the warm garlic butter until the mixture is fully combined.
Arrange 6 brioche hamburger buns, split face-up onto your prepared parchment baking sheet to create exactly 12 individual halves.
Spread the seasoned garlic butter mixture evenly across the open faces of all 12 bun halves using a pastry brush or the back of a spoon.
Bake at 375°F for 6–7 minutes until the edges of the brioche just start to turn a light golden brown.
Remove from oven and immediately sprinkle ¼ cup freshly grated Parmesan cheese evenly over the top of the hot, toasted buns.
Broil on high for 1–2 minutes until the cheese layer is completely melted, bubbly, and beautifully golden brown.