Whisk the broth, hoisin sauce, soy sauce and peanut butter together in a medium bowl.
Heat 1 teaspoon of the oil in a 12-inch skillet over high heat until shimmering. Add pork and cook, stirring to break up the meat, until thoroughly cooker, about 3 minutes. Transfer to plate and set aside.
Add 1 tablespoon oil to the empty pan, swirl to coat the pan, and add the eggplant. Cook, stirring frequently, until browned, about 3 minutes.
Clear a space in the center of the pan and add garlic, ginger, pepper flakes, and remaining 1 teaspoon oil. Cook until fragrant, about 10 seconds.
Stir in pork and sauce. Cover, reduce the heat to medium, and cook until the sauce has thickened and the eggplant has softended, about 4 minutes.
Garnish with the scallions and serve
