Make the sauce: Whisk cornstarch and water together in a small bowl. It will be a milky liquid. This is for thickening the sauce. Combine the cornstarch mixture, brown sugar, honey, soy sauce, vinegar, garlic, and ginger together in a small saucepan over low heat. Allow to simmer while whisking occasionally. Bring to a boil and allow to boil for 1 minute, then remove from heat. Place shrimp into a heat-proof bowl, then pour half of the sauce on top. Give it a stir and set aside for 10 minutes, uncovered at room temperature. If cooking later on, you can marinate the shrimp for up to a day. Keep it covered and refrigerate. (I usually begin cooking rice to have on the side at this point, but that's optional.) Heat oil in a skillet over medium-high heat. Add the snap peas, then stir and cook until crisp-tender, about 3-4 minutes. Add the shrimp, then the remaining half of the teriyaki sauce. Stir and cook until shrimp is pink and cooked through, about 2-3 minutes. Serve with rice and a sprinkle of sesame seeds if desired. Leftovers keep well in the refrigerator for a few days.