In a large bowl, whisk together the flour, baking powder, and salt. In a separate bowl, combine the cold sparkling water and beaten egg. Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Do not overmix; a few lumps are okay.
Heat about 2 inches of vegetable oil in a deep skillet or pot over medium-high heat until it reaches 350°F.
Dip the sweet potato slices into the batter, allowing the excess to drip off. Carefully place them in the hot oil, frying in batches to avoid overcrowding.
Fry the sweet potatoes for about 3-4 minutes or until they are golden brown and crispy. Use a slotted spoon to transfer them to a paper towel-lined plate to drain excess oil.
Serve immediately with your choice of dipping sauce.