Heat the sesame oil in a large skillet over medium-high heat.
Add the cubed chicken, curry powder, and a pinch of salt; cook, stirring occasionally, for about 3 minutes to brown the chicken.
Add the red curry paste, tomato paste, butter, chopped garlic, and chopped ginger to the skillet and cook for 2 more minutes, stirring to combine.
Add the chopped broccoli, then pour in the canned coconut milk and season with salt; bring to a gentle simmer.
Simmer for 5–10 minutes, stirring occasionally, until the chicken is cooked through and the sauce has thickened.
Stir in the chopped cilantro and remove from the heat.
To make the mango topping: in a bowl combine the diced mango, 1 cup chopped cilantro, toasted sesame seeds, lemon juice, and a pinch of chili flakes; mix and serve alongside the curry.
Serve the chicken curry over rice with the mango topping on the side.