How to Make It
Butter a large rimmed baking sheet. In a small saucepan, combine the sugar with 1/4 cup of the water and bring to a simmer, stirring until the sugar dissolves. Cover and simmer the syrup over moderate heat for 1 minute. Uncover and simmer over moderately high heat, without stirring, until an amber caramel forms, about 6 minutes. Remove from the heat and stir in the sesame seeds with a skewer or chopstick. Immediately pour the caramel onto the baking sheet and tilt carefully to spread the caramel in a thin layer. Using 2 forks, pull and stretch the caramel into as thin a layer as possible. Let cool completely, about 20 minutes.
Using a spatula, pry the sesame praline from the sheet and break it into shards. Transfer half of the shards to a food processor and pulse to a fine powder. Reserve the remaining shards for garnish.
Line a 9-by-4 1/2-inch loaf pan with plastic wrap. Spoon the sorbet into the loaf pan and smooth the surface. Freeze until firm, about 30 minutes.
In a medium microwave-safe bowl, sprinkle the gelatin over the remaining 3 tablespoons of water and let soften for 2 minutes. Microwave on high power for 10 seconds, just until melted. Whisk in the cream of coconut.
In a large bowl, using an electric mixer, beat the heavy cream with the coconut extract at high speed until firm. Add the cream of coconut mixture and beat at low speed just until combined.
Carefully lift the sorbet rectangle out of the loaf pan and return to the freezer. Line the loaf pan with a double sheet of plastic wrap, leaving at least 2 inches of overhang on all sides. Pour one-third of the coconut mousse into the loaf pan and sprinkle with half of the sesame praline powder. Unwrap the sorbet rectangle. Ease it into the loaf pan and sprinkle with the remaining praline powder. Using a rubber spatula, gently spread the remaining coconut mousse on top. Fold the overhanging plastic wrap over the top of the terrine and freeze until it is firm, at least 6 hours or overnight.
Carefully unmold and unwrap the terrine. Using a hot knife, cut the terrine into slices. Garnish the slices with the praline shards, diced mango and fresh coconut and serve.