Place chicken broth in small saucepan. Add lemon grass, ginger and lime zest. Bring to a simmer; cover and cook for 10 minutes. Stir in fish sauce (if using); set aside.

While the chicken broth mixture is simmering, warm vegetable oil over high heat in a large, non-stick skillet or wok. Toss chicken chunks in cornstarch. Add chicken to heated oil in two batches; saut until golden brown, about 5 minutes. Remove chicken, place on plate and set aside. Add pineapple chunks to pan and saut quickly, about 2 minutes.

Return chicken to skillet or wok. Pour chicken broth into pan through strainer, separating out lemon grass, ginger and lime zest pieces. Discard these pieces. Stir over high heat until sauce reaches simmer, about 1 minute. Reduce heat to low and simmer for 4 minutes, until sauce has thickened lightly. Add salt. Sprinkle with cilantro and mint. Serve immediately over rice or noodles.