Preheat oven to 350F. Grease the bottom and sides of an 8-inch cake pan with parchment paper. Set aside.
In a bowl of a standing mixer with a whisk attachment, beat egg whites on medium speed until foamy. Add 2 tablespoons sugar and continue beating until stiff peaks form. Scrape the meringue in another bowl. Set aside.
In the same mixing bowl, beat egg yolks and 1/4 cup sugar on high speed until thick and light in color, about 5 minutes. Add coffee and oil. Mix for another minute.
Sift flour, cocoa powder, baking powder, salt and the remaining 2 tablespoons sugar over top of egg yolk mixture. Fold the flour mixture in the egg mixture, followed by half of the meringue.
Fold the rest of the meringue into the batter until just incorporated. Pour the batter into the prepared pan and smooth the top.
Bake 20 to 25 minutes or until the centre of the cake springs back when gently pressed.
Remove the cake from the oven and let cool in pan for 5 minutes. Run a thin knife around the cake edge and place the cake on a wire rack to cool completely.
To prepare mango mousse, combine mango puree, sugar and lime juice in a bowl. In a small saucepan, sprinkle gelatin over the water, let stand for 5 minutes. Then dissolve over medium-low heat, and once dissolved, remove it from the heat and stir it into the puree mixture. Let cool completely.
Whip the heavy cream until it becomes stiff. Add the cream to the mango mixture and fold with a spatula until it becomes one liquid.
To assemble, line the bottom and sides of an 8-inch springform pan with parchment paper.
Place in the chocolate cake layer. Then pour the mango mixture over the cake. Juggle to lightly level the top.
Chill in the refrigerator overnight.
To prepare ganache, place chocolate and heavy cream in a medium heatproof bowl over a pot of simmering water. Stir the mixture using a wooden spoon until melted and smooth, about 5 minutes. Remove from heat and let cool slightly.
Remove cake from the refrigerator. Transfer cake from cake pan to a serving plate. Peel off the parchment papers.
Pour ganache over the top edges of the cake. Use an offset spatula to gently push the ganache over the sides of the cake to create the dripping effect.
Garnish with whipped cream, fresh berries and chocolate shavings, if preferred.