Begin by pouring about 2 inches of vegetable oil into a deep pot or fryer. Heat the oil to a temperature of 350°F (175°C) over medium heat.

In a large mixing bowl, combine all-purpose flour, cornstarch, baking powder, and salt. Whisk gently until blended.

Slice the Japanese sweet potatoes into evenly sized pieces. Dip each slice into the batter, ensuring full coating.

Carefully drop the battered sweet potato slices into the hot oil in small batches. Fry for about 3-4 minutes until golden brown.

Once golden, use a slotted spoon to remove the tempura from the oil and place on paper towels to drain excess oil.