Rinse and peel the sweet potatoes. Slice them into thin rounds of about ¼ inch.
Place oven racks on lower and upper thirds. Preheat oven to 400 degrees.
In a small saucepan melt the butter over low heat. Once it melts pour in the olive oil, combine and remove from the heat.
Brush the butter/oil mix onto the bottom of two large baking sheets. Place the sweet potato rounds in a single layer. Brush the tops with more of the butter/oil mix. In a small bowl, combine the ground chile, the salt and the sugar. Sprinkle the sweet potatoes generously with ground chile blend. Flip rounds on to the other side and sprinkle generously with ground chiles. Place in the oven for about 20 minutes, or until they have started to brown. Remove baking sheets. With a set of tongs or a pair of forks, flip the sweet potato rounds. Place back in the oven for 12 to 15 minutes more, until they have browned on both sides. Don't let them burn.
Remove them from the oven. As they cool a little, their edges will crisp a bit more. Serve hot or not: either way they are delicious!