Place a potato between the handles of 2 wooden spoons or chopsticks. Slice the potato into thin slices, leaving 1/4 inch at the bottom unsliced; the spoons or chopsticks will prevent you slicing all the way through. Repeat with the remaining potatoes.
Mix together the butter, brown sugar and cinnamon with a pinch of salt and pepper in a small bowl.
Place the sweet potatoes on a baking sheet. Using a spatula, spread the butter mixture all over the potatoes. Bake until tender and golden, 40 to 45 minutes. Transfer to a platter and garnish with the sage and rosemary.
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