Plate dates and hot water in a bowl and soak for at least 30 minutes, until soft. Remove the pits and discard.
In a food processor or blender, puree soaked and pitted dates and their soaking water, soy sauce, rice wine vinegar, garlic and ginger. Set aside.
Heat oil in a wok or large skillet and set on medium-high heat. Add the onions and sauté 2-3 minutes, until softened and starting to brown.
Combine flour, salt, pepper, and chili powder in a shallow bowl or pie plate. Toss chunks of chicken breast in the flour mixture until coated. Set aside.
Add coated chicken breast chunk to the hot wok and sauté until the outside is browned. Pour date sauce and chicken broth in to the pan with the chicken and reduce the heat to medium; allow to simmer for 8-10 minutes, covered. Add mango chunks and simmer 5 additional minutes, covered.
Serve over rice and garnish with sesame seeds and sliced green onion.