Put the chili pepper, ginger, and garlic in a food processor and pulse until finely minced. Add about 1/3 of the cilantro and process until it becomes a rough paste.
Heat the oil in a large skillet or frying pan over medium-high heat until hot and almost smoking. Add the chicken, season with salt and black pepper, and cook for about 2 minutes, stirring occasionally, until browned on all sides.
Stir in the cilantro paste and cook for 1 minute. Add the peanut butter, stock or broth, and yogurt, and stir to combine. When the sauce is gently bubbling, turn down the heat to medium and cook until the chicken is cooked through and the sauce is slightly thickened, 7-10 minutes. If the sauce gets too thick, add a splash more stock.
When the curry is done, stir in the remaining cilantro, leaving a handful aside for garnish, if desired. Serve with rice, scattering the garnishing cilantro on top.