In a large bowl, sift together the flour, baking soda, and salt; set aside.
In the bowl of an electric mixer, cream the butter and the sugar until light and fluffy. Add the cooled chocolate and vanilla and beat for 3 minutes to incorporate.
Beat in the eggs one at a time. Scrape down the sides of the bowl and beat for another 3 minutes.
Gradually mix in the dry ingredients in 3 batches, alternating with the water. Beat for 1 minute after each addition to incorporate the ingredients.
Line the baking sheet with parchment paper, pour in the batter and smooth the surface with a spatula. Bake for 20 minutes, until tester inserted in the cake comes out clean.
Leave to cool. With the 3 cookie cutter cut 4 circles, brush with the liqueur and set aside.
To make the coconut mousse, bring the first amount of milk and the coconut to a boil. Drain through a fine-mesh sieve, squeezing the coconut well and reserving the milk.
Bring reserved coconut milk to a boil. Mix the cornstarch with 2 tbsp of cold milk and add to the hot milk. Cook well until start to thicken. Cool.
Dissolve the gelatin in remaining 2 tbsp of cold milk. Let stand for 2 minutes. Heat the milk just until gelatin dissolves. Cool slightly.
Combine cooled coconut milk with gelatin and liqueur. Whip the cream and fold it gently inches.
Line two 5-inch ramekins with acetate and pour in the coconut mousse. Cover with plastic wrap and freeze.
To make a chocolate mousse, combine sugar with cornstarch and eggs. Mix until no lamp remains. Heat the milk in a sauce pan and pour gradually into egg mixture. Return to the pan and cook, stirring constantly until start to thicken. Do not let to boil. Remove from the heat, cover tightly and cool.
Dissolve gelatin in the milk. Let stand for 2 min and heat until gelatin is dissolved.
Whip chilled pastry cream, add gelatin and melted chocolate. Whip the whipping cream until soft peaks and fold it inches.
Line four 4-inch springform pans with acetate. Place the cake circles on the bottom. Unwrap frozen coconut mousse and place them on top of the cakes. Pour chocolate mousse over the coconut mousse, dividing evenly between the pans.
Cover with plastic wrap and chill for at least 6 hours.