Choose 4 (5-ounce) desired containers.
To make the gelee, in a small pot over medium heat, add the mango puree and 2 tablespoons water. Bring to a simmer and remove from heat.
Meanwhile, in a small bowl, add 2 tablespoons cold water and sprinkle with gelatin powder. Let it sit for 1- 2 minute.
Stir the gelatin into the mango puree. Mix until dissolved, about 3 – 5 minutes. Taste and add sugar if needed. Let it cool for 10 minutes at room temperature. Stir in the mango cubes.
Carefully spoon ½ of the mango gelee into the containers evenly. Transfer the containers to the fridge. Cover the remaining gelee with a plastic wrap. Make sure the wrap is touching the surface of the gelee. Cool at room temperature.
For the chocolate mousse, in a large heatproof bowl, whisk together egg yolks, coffee, salt, and ¼ cup sugar. Set the bowl over a pot of simmering water. Make sure the water is not touching the bowl. Whisk constantly until the sugar has dissolved, the mixture is lighter in color and doubled in volume, about 5 minutes. Remove the bowl from heat. Whisk in the chocolate until melted and smooth. Let it cool to room temperature, whisking occasionally.
In a large mixing bowl with an electric mixer fitted with a whisk attachment, beat the egg whites until foamy with uniform small bubbles. Add 1½ tablespoons sugar and beat on high speed until stiff peaks form.
Fold the egg whites into the chocolate mixture in 2 addictions. Fold in the whipped cream until just blended. With a spoon or piping bag, divide the chocolate mousse into the 4 containers. Return the mousse back to the fridge and chill for 30 minutes.
30 minutes later, transfer the remaining mango gelee into a piping bag. Insert into the center of the mousse and pipe about 1 teaspoon of gelee into each cup. Top each cup of mousse with another teaspoon of gelee. Cover the containers with plastic wrap and put back into the fridge. Let it set for at least 2 hours.
If making chocolate hearts, line a small baking sheet with parchment paper.
In a small microwave bowl, add chocolate chips. Heat in the microwave for 1 minute with medium power. Stir and heat again for another 30 seconds. Stir again. Repeat until the chocolate is almost melted. Stir until smooth. Transfer the melted chocolate to a piping bag. Cut a tiny hole and pipe out hearts on the prepared parchment paper. When finished, place the baking sheet in the freezer for 5 minutes to harden.
To assemble, decorate the mousse with extra mango cubes, chocolate pearls and chocolate hearts if using. Serve.