Boil sweet potatoes in water to cover for 10 minutes. Remove and allow to cool. Peel potatoes and cut into 1/4 by 1/4-inch thick batons.

Meanwhile in a 5-quart pot or Dutch oven fitted with a candy or deep-frying thermometer, (or in an electric deep fryer), heat oil over medium-low heat until the thermometer registers 300 degrees F. Make sure you have at least 3 inches of space between the top of the oil and the top of the pan, as fries will bubble up when they are added.

Increase the heat to medium-high and add fries, a handful at a time, to the hot oil. Fry, stirring occasionally, until potatoes are soft and limp and just begin to brown, about 3 to 4 minutes. Using a skimmer or slotted spoon, carefully remove fries from the oil and set aside to drain on paper towels. Let rest for at least 10 minutes or up to 2 hours.

When ready to serve the fries, reheat the oil to 350?F. Transfer the blanched potatoes to the hot oil and fry again, stirring frequently until browned and puffed, about 1 minute. Transfer to paper lined platter and allow to drain for a minute.

Combine the brown sugar, salt, cayenne and cinnamon in a large bowl. Add the fries and toss to coat. Serve immediately.