Preheat the oven to 400°F. Line a baking sheet with foil or parchment paper.
Scrub the sweet potatoes thoroughly and pat them dry.
Place one sweet potato on a cutting board with a wooden spoon or chopstick along each side. Using a sharp knife, cut ?-inch slices across the potato, stopping before cutting all the way through. The wooden spoons or chopsticks will help prevent the knife from cutting through the bottom.
Repeat with the remaining sweet potatoes and place them on the prepared baking sheet.
In a small bowl, stir together the melted butter, olive oil, salt, brown sugar, and cinnamon.
Brush half of the butter mixture over the sweet potatoes, making sure to get some between the slices.
Loosely cover the potatoes with foil and bake for 30 minutes.
Remove the foil and brush the remaining butter mixture over and between the slices.
Bake uncovered for an additional 30–35 minutes, or until the sweet potatoes are tender and cooked through.
Serve warm and enjoy.
Notes
Refrigerate the leftovers in an airtight container for up to 5 days.