Place the avocado, stock, yogurt, lime juice, 1/2 tsp. of the cumin, cayenne and sugar in a food processor. Pulse until smooth; season with salt and pepper. Transfer to a bowl. Cover and store in the fridge. Preheat an indoor or outdoor grill to medium-high. Brush the chicken with the olive oil; season with the remaining cumin, paprika, salt and pepper. Grill 3-4 minutes per side, or until cooked through. Serve the chicken on a pool of the sauce; top with the chopped tomato and cilantro and serve.