In an electric skillet or deep fryer, heat oil to 375°. Combine flour, cornstarch, salt and baking powder. Stir in club soda just until blended (batter will be thin). Dip florets, a few at a time, into batter and fry until cauliflower is tender and coating is light brown, 8-10 minutes. Drain on paper towels.
For sauce, whisk together the first 6 ingredients; whisk in cornstarch until smooth.
In a large saucepan, heat canola oil over medium-high heat. Add chiles; cook and stir until fragrant, 1-2 minutes. Add white part of onions, garlic, ginger and orange zest; cook until fragrant, about 1 minute. Stir soy sauce mixture; add to saucepan. Bring to a boil; cook and stir until thickened, 2-4 minutes.
Add cauliflower to sauce; toss to coat. Serve with rice; sprinkle with thinly sliced green onions.