Pierce the sweet potatoes with a fork or knife point in several places, then bake in a pre-heated 400 degree oven for about 1 hour, or until tender. Set aside.
Slice the chicken breast into small cubes, approximately 1 inch thick.
Heat the oil in a deep pot, carefully add the chicken pieces. Allow to cook undisturbed on once side for about 3-4 minutes, or until golden brown. Flip the chicken and allow to brown on the other side. Remove the chicken to a bowl and set a side.
To the hot pan, add the onions and cook, stirring occasionally, on medium-low heat until they soften and start to take on color.
Add the dried spices, stir and cook for 1-2 minutes.
Stir in the tomatoes, sugar, and water. Raise the heat to high and bring to a boil. Stir, cover, reduce heat and simmer gently for 20 minutes.
Add the reserved chicken, chickpeas, chopped kale and simmer until the greens are tender, 3-8 minutes. Then turn off the heat and let cool for about 10 minutes before serving.
Cut each sweet potato in half.
To serve, top a sweet potato half with the curry, a spoonful of yogurt, and chopped cilantro or mint (if using).