Melt the bittersweet chocolate, butter, and milk in a heatproof bowl over a pot of simmering water. Once melted, remove from heat and let cool slightly.

Beat the egg whites with a pinch of salt until soft peaks form. Gradually add sugar, continuing to beat until stiff peaks form.

Mix the egg yolks and vanilla into the cooled chocolate mixture. Gently fold in a third of the egg whites to lighten the mixture, then carefully fold in the remaining whites.

Pour the batter into a greased and parchment-lined round cake pan. Bake in a preheated oven at 175°C (350°F) for about 25 minutes. Let cool completely.

Melt the 150g of chocolate with 90ml of heavy cream, stirring until smooth. Cool to room temperature.

Whip the 300ml of cold heavy cream with sugar until it forms soft peaks. Fold in the cooled chocolate mixture until no streaks remain.

Dissolve cocoa powder and sugar in hot water, stirring until smooth. Let cool.

Mix cocoa powder, sugar, and water in a saucepan. Bring to a boil, then simmer for about 5 minutes, stirring occasionally.

Bloom the gelatine in cold water, then add it to the hot cocoa mixture along with the heavy cream. Stir until the gelatine is fully dissolved and the mixture is smooth. Cool until it thickens slightly.

Slice the cooled cake horizontally to create two layers. Place the bottom layer on a serving plate and soak with half of the cocoa syrup.

Spread the chocolate cream filling over the bottom layer, then top with the second cake layer. Soak the top layer with the remaining cocoa syrup.

Pour the chocolate glaze over the cake, spreading it to cover the top and sides. Refrigerate until set.