In a large bowl, season shrimp with salt and pepper, to taste. Stir in cornstarch and gently toss to combine.
Preheat the oven to 400°F.
Coat a rimmed baking sheet with cooking spray,
then arrange the shrimp in a single layer on the baking sheet. Bake for 10 minutes.
Pour the marinade into a saucepan. Bring to a boil over high heat and cook until thick and syrupy, about 2 minutes. Brush shrimp with thickened marinade and serve.
Cooking the shrimp in the pan:
Heat the olive oil in a large pan to medium-high heat. Add the shrimp and cook 2-3 minutes per side, until the shrimp is pink and cooked through.
Scatter garlic over shrimp and cook, shaking pan often, 1 minute. Add sugar, fish sauce, 3 tablespoons water, pepper; toss to combine. Cook, shaking pan often, 1 minute.
Serve immediately, garnished with cilantro.