Heat vegetable oil in a large skillet or wok over medium heat.
- Add Panang curry paste and sauté for 1–2 minutes until fragrant.
- Pour in half the coconut milk and stir until combined with the curry paste. Simmer until the oil begins to separate slightly.
- Add the sliced beef and cook until just browned, about 3–4 minutes.
- Stir in the remaining coconut milk, fish sauce, sugar, kaffir lime leaves, and peanut butter (if using). Simmer for 5–7 minutes, until beef is tender and the sauce thickens slightly.
- Taste and adjust seasoning with more fish sauce, sugar, or chili if desired.
- Stir in fresh basil just before serving.
- Serve hot over jasmine rice, garnished with sliced chili or bell pepper.