Preheat oven to 450° Fahrenheit. Line baking sheet with aluminum foil and set aside.
Place 1 sweet potato horizontally on cutting board. Position 1 wooden spoon handle or 1 skewer against top side of sweet potato, and position other spoon or skewer against bottom side of sweet potato.
Position knife perpendicular to sweet potato. Place knife against end of sweet potato, then work toward other end of potato, making slices every ⅛ inch. Use spoon handles or skewers as bumpers to catch knife and prevent slicing completely through potato.
Once potato has been thinly sliced from one end to other, set sliced potato on foil-lined baking sheet. Repeat process with remaining sweet potatoes.
Add butter, garlic, maple syrup, rosemary, cinnamon, and salt to small bowl. Whisk or stir until ingredients are fully incorporated.
Use basting brush to evenly apply butter mixture to sweet potatoes, brushing butter down between slices whenever possible.
Place baking sheet in preheated oven. Bake sweet potatoes 25 minutes, then carefully remove baking sheet from oven. Place fork under center of potato. Gently lift potato up slightly, not actually removing potato from pan but simply allowing slices to fan apart slightly. Repeat with remaining sweet potatoes, then return baking sheet to oven.
Bake sweet potatoes 20 additional minutes or until potatoes are cooked through and tender. Carefully remove baking sheet from oven. Sprinkle salt and pepper over potatoes as desired and serve warm.