Toss the chicken with the scallions, soy sauce, sugar, 1 teaspoon of the cornstarch, 1 teaspoon of the salt and the sesame oil. Marinate at room temp 15 minutes.
Mix the remaining cornstarch with 1/3 cup water to make a slurry.
Heat a large nonstick pot/wok or skillet over high heat.
Add 1 tablespoon of the oil and heat.
Add the broccoli and stir-fry for 30 seconds. Add garlic.
Add 2 tablespoons of water and sprinkle on 1/4 teaspoon salt.
Stir-fry until the broccoli is bright green but still crisp, about 2 minutes. Transfer to a plate.
Get the skillet hot again and then heat 2 more tablespoons oil.
Add the chicken and stir-fry until the chicken gets a bit brown, 3 mins.
Add the hoisin sauce and return the broccoli to the pan.
Toss!
Add slurry and bring to a boil.
If too thick, add a bit more water.
Taste and add salt if needed.
Serve over rice with scallions to garnish.