In a medium saucepan, combine ketchup, brown sugar, vinegar, lemon juice, water, and cornstarch. Stir and place over medium heat. Continuously stir until boiling, then let simmer for about 5 minutes until thickened. Remove from heat and cool.

In a large mixing bowl, whisk together all-purpose flour, cornstarch, baking powder, sugar, and coarse kosher salt. Gradually add cold water, stirring until thick and smooth and free of lumps.

Preheat oil in a deep pot to 375°F (190°C). Dip chicken chunks in batter, then drop into the hot oil in batches. Fry for 3-4 minutes on each side until golden brown. Use a slotted spoon to transfer to a paper towel-lined plate.

Arrange the chicken balls on a serving platter. Serve with the sweet and sour sauce for dipping, optionally garnish with sesame seeds or green onions.