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Season the chicken with salt and black pepper.
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Add the egg, cornstarch, flour, and water. Mix until a smooth batter forms and evenly coats the chicken pieces.
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Heat oil to 350°F (175°C) and fry the chicken until golden brown and cooked through, about 5–6 minutes. Transfer to a wire rack.
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Make the orange sauce by combining the orange juice, soy sauce, sugar, rice vinegar, garlic, ginger, and orange zest in a saucepan. Bring to a simmer.
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Mix the cornstarch and water to create a slurry. Stir into the sauce and cook until thick, glossy, and able to coat the back of a spoon. Remove from the heat and stir in the butter.
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Toss the fried chicken with enough sauce to coat every piece generously. Reserve about 200g of the chicken for the topping and use the remainder for the filling.
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Transfer the dough to a well-oiled 9×13-inch pan and gently stretch it into a rectangle. Spread the orange chicken filling evenly over the surface and scatter the sliced green onions on top.
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Fold the dough in thirds like a letter, then fold it in half to fully enclose the filling. Turn the dough seam-side down and gently tuck the edges underneath to form a neat package.
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Cover and let proof for 2 hours, or until the dough is noticeably puffy and has spread to fill most of the pan.
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Drizzle with olive oil and dimple the dough with your fingertips before baking.
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Bake the focaccia until golden brown.
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Immediately after baking, brush the surface generously with orange sauce. Top with the reserved orange chicken and drizzle with additional sauce until the chicken is glossy and well coated. Sprinkle with sesame seeds and broil for 2–3 minutes.
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Finish with fresh green onion, orange segments, and orange zest before serving.
