For the meatballs: Preheat the oven to 375 degrees F and line a sheet tray with parchment paper.
Beat the egg in a large bowl with a fork, then add the hoisin sauce, soy sauce, garlic, ginger and scallions. Stir in the panko, then add the ground beef, tearing into small pieces before adding to the bowl.
Mix together by hand until completely combined, then use a 1-tablespoon ice cream scoop or spoon to make meatballs and place on the prepared baking sheet. Shape the meatballs into smooth round balls. Bake until cooked through, 15 to 17 minutes.
For the sauce: Whisk to combine the dark brown sugar, soy sauce, hoisin sauce, sesame oil, garlic, ginger, red pepper flakes and 1 cup water in a large skillet. Bring to a simmer and cook until the sugar is dissolved and the sauce is thickened, 6 to 8 minutes.
Whisk the cornstarch with 1 tablespoon water in a small bowl until smooth. Stir into the simmering sauce, then add the cooked meatballs and cook until the sauce is thickened and the meatballs are coated, about 30 seconds.
For the rice and garnishes: Serve the meatballs over rice and garnish with sliced scallions and toasted sesame seeds.