Preheat the oven to 425°F* (220°C). Line a baking sheet with parchment paper or foil.
Place each sweet potato between two chopsticks or wooden spoons on a cutting board. Slice crosswise into thin slits about ⅛ inch apart, being careful not to cut all the way through. Repeat for all sweet potatoes.
In a small bowl, stir together melted butter, garlic powder, kosher salt, and black pepper until well combined.
Place the sliced sweet potatoes on the prepared baking sheet. Using a pastry brush or spoon, generously coat each sweet potato with the garlic butter mixture, brushing some into the slits. Reserve a little of the mixture for later.
Bake the sweet potatoes for 30 minutes.
After 30 minutes, carefully use a knife to gently pull the layers apart. Brush again with some more of the reserved garlic butter, saving the rest for serving.
Return the potatoes to the oven and bake for another 15-20 minutes, or until the sweet potatoes are crispy on the outside and tender on the inside. (Cooking times may vary depending on the size of the potatoes you are using.)
Remove from the oven. Brush with the remaining garlic butter and garnish with fresh rosemary or thyme if using. Sprinkle with a little extra kosher salt or black pepper to taste.