Add the water, soy sauce, rice vinegar, brown sugar, cornstarch, garlic powder, sesame oil and red pepper flakes to a bowl. Whisk well to combine and set aside.
Heat a large nonstick skillet over medium high heat. Add the butter and let melt. Add the shrimp and season with salt and pepper. Cook until they just turn pink, about 3-4 minutes. Transfer to a plate or bowl.
Increase the heat to high. Give the reserved sauce a good whisk and add to the skillet. Let simmer until thickened, about 3 – 4 minutes.
Turn the heat down to medium. Add the shrimp and toss to coat. Let cook for 30 seconds. Garnish with sesame seeds.