Cook rice according to package directions, substituting half the water with coconut milk and adding 1/4 cup sugar.
Check during cooking and add more water a few tablespoons at a time if needed.
It will be sticky without any liquid left at the bottom when done.
While rice is cooking, boil the rest of the coconut milk with the remaining 1/4 sugar. Reduce to a simmer and continue cooking until the sauce becomes thick and syrupy.
Place two slices of mango over each hole in the sushi mold. Spoon half the rice mixture into the mold on top of the mango slices and press with the lid.
Remove the lid, flip the mold over and press gently on mango slices to release. Place on platter, drizzle with warm coconut sauce and serve warm.