Pre-heat the oven to 375. Lightly mist 12 cups in a standard muffin/cupcake tin with cooking spray and set aside.
Pour the sesame oil in a large skillet or saute pan and bring over medium-high heat. Add the ground chicken and cook, stirring and breaking up into small pieces, until the chicken is cooked through. Add the garlic and ginger to the meat and stir together. Cook for another minute and then add the mushrooms, cabbage, half the scallions and the black pepper. Stir together and cook for 2- 3 minutes until the veggies are softened. Remove from heat. Add the soy sauce, 1 tablespoon of the Hoisin sauce, and the sriracha and stir to combine.
Push a wonton wrapper into the bottom of each of the sprayed cups in the muffin tin. Using about half of the chicken mixture, spoon evenly into the wonton wrappers. Press another wonton wrapper on top and fill with the remaining chicken mixture.
Bake for 18-20 minutes until the wontons are golden brown and the contents are heated through. Remove the muffin tin from the oven and allow to cool for 2-3 minutes before removing from the tin. Drizzle the remaining tablespoon of Hoisin sauce onto the tops of the wonton cups and sprinkle with the remaining sliced green onions.