In a mixing bowl, whisk together all-purpose flour, cornstarch, sugar, baking powder, and coarse kosher salt. Gradually add cold water, stirring until thick yet pourable.

Pour enough oil into a deep pan to create about 1 inch of depth and heat it over medium-high heat until it shimmer but not smoke, aiming for 350°F (175°C).

Slice chicken breasts into ¼-inch strips and dip each strip into the batter, ensuring full coverage before draining excess batter.

Lower battered chicken strips into hot oil carefully, frying for 2-3 minutes on each side until golden brown and crispy. Drain on paper toweling.

In a saucepan, combine ketchup, brown sugar, vinegar, lemon juice, and water. Add mixed cornstarch and heat over medium until sauce thickens.

Serve warm chicken fingers with sweet and sour sauce for dipping.