Toss chicken pieces with soy sauce and let stand at room temperature while you make the sauce.
Heat oil in a large pot over medium heat. Cook the garlic until fragrant, then add the orange juice. In a bowl or measuring cup, whisk the cornstarch with the soy sauce until smooth, then stir into orange juice mixture. Add the honey, sugar, vinegar and salt. Bring to a boil, then reduce heat and cook a few minutes more until sauce is thick and glossy. Remove from heat.
Heat 1 inch of oil in a large wok or dutch oven. In a medium bowl, whisk together the flour, cornstarch, and baking powder. Add the water, stir gently to combine, then add the chicken and any liquid in the bowl. Stir to evenly coat the chicken.
When oil is shimmering hot (or 350ºF), add some chicken pieces using tongs - work carefully and in batches so that the pieces don't overlap. Cook chicken, turning once, until pieces are deeply golden brown and reach 165º in the centre. Transfer to a paper-towel-lined plate and continue with next batch until all chicken is cooked. Just before serving, gently reheat the sauce and add the chicken, tossing until coated. Top with green onions to serve.