How to Make It
Preheat the oven to 350°. On a rimmed baking sheet, spread the sweet potato cubes in an even layer and season with salt. Roast until almost tender, about 30 minutes. Let the sweet potato cool.
Meanwhile, in an ovenproof skillet, bake the cinnamon sticks until fragrant, about 10 minutes. Crush the cinnamon sticks, transfer to a blender and blend until a fine powder forms. Sift the cinnamon powder through a fine sieve into a small bowl, then stir in the granulated sugar and 1 teaspoon of salt. In another small bowl, stir the brown sugar with 11/2 tablespoons of salt.
In a medium, heavy saucepan, heat 2 inches of oil until it reaches 375° on a candy thermometer. Line a baking sheet with paper towels. In a small bowl, whisk 1/2 cup plus 2 tablespoons of the flour with the baking powder and cornstarch. Transfer 1⁄2 cup of the dry ingredients to a medium bowl, then whisk in the seltzer just until smooth. (Discard the rest of the dry ingredients.) Place the remaining 1/2 cup of flour in a small bowl.
Working in batches, toss the sweet potato cubes in the flour and tap off the excess. Using a fork, dip the cubes in the tempura batter, allowing the excess to drip back into the bowl. Add the battered sweet potato cubes to the hot oil and fry until golden, about 3 minutes per batch. Drain on the paper towels. Serve the sweet potato tempura with the two sugars for dipping and shots of bourbon on the side.