Add soy sauce, beef broth, brown sugar, hoisin sauce, ginger, red pepper flakes, and black pepper to the skillet.
Stir to combine all the ingredients.
Dissolve cornstarch in 2 tablespoons of cold water, then add it to the ground beef mixture.
Stir and bring the mixture to a gentle boil.
Once the sauce has thickened, add the cooked noodles and mix them in to combine.
Allow the noodles to sit in the sauce for 2-3 minutes to absorb some of the flavors.