Whisk cornstarch and water in a small bowl and set aside. Cook garlic, ginger, and sesame seeds with sesame oil in a small saucepan or wok over medium heat for 3 to 4 minutes until browned. Add the hoisin, soy sauce, orange juice and zest, vinegar, sugar, and chili garlic paste then bring to a boil over medium high heat, stirring often. Continue boiling until the sauce is just starting to thicken a little, about 10 minutes. Pour in water/cornstarch while stirring and stir until thick. Remove from heat and stir in the tahini until sauce is smooth. Pour into a bowl for serving and garnish the top with sesame seeds. You can serve the sauce hot or chill in fridge for a few hours and serve cold.