1-First Step: Preparation and soaking gelatin. Line a 16 cm (6.3-inch) cake tin with greaseproof paper for easy removal. Soak gelatin leaves or powdered gelatin in 20 ml cold water for 10-15 minutes until fully softened to prepare for mousse layers.

2-Second Step: Prepare the strawberry puree. Blend 180 grams of fresh strawberries with 40 grams sugar and 10 ml lemon juice if desired until smooth. Gently heat the puree to dissolve any gelatin fully, stirring carefully, then let it cool to room temperature to avoid premature gel formation.

3-Third Step: Make the chocolate mousse. Heat 60 grams of whipping cream and melt the 90 grams chocolate in a bowl over simmering water. Dissolve the bloomed gelatin into the slightly cooled chocolate mixture. Whip 125 grams cream to soft peaks, carefully folding half into the chocolate to keep it light. Combine all gently to create the chocolate mousse.

4-Fourth Step: Assemble the first layer. Quickly soak 25 grams of ladyfingers in 100 ml whole milk, then press them evenly into the base of the lined cake tin. Pour the chocolate mousse over the ladyfingers and chill the cake until set, about 30-60 minutes.

5-Fifth Step: Create the strawberry mousse layer. Whip the remaining 125 grams cream to soft peaks. Fold into the cooled strawberry puree, then layer a fresh set of soaked ladyfingers gently on top of the set chocolate mousse. Pour the strawberry mousse over and chill again until firm.

6-Sixth Step: Prepare the vanilla custard mousse. Whisk 1 egg yolk with 25 grams sugar until pale. Stir in 10 grams corn starch or flour. Gradually add 125 ml warm whole milk and cook over medium heat, stirring constantly until thickened custard forms. Dissolve gelatin into the warm custard and allow it to cool. Whip 110 grams cream to soft peaks and fold gently into the custard mixture.

7-Seventh Step: Finish assembly and chill. Place another layer of soaked ladyfingers atop the strawberry mousse, pour the vanilla custard mousse evenly over the top, and chill the entire cake for 2-4 hours or overnight until completely firm and mousse layers are set.

8-Final Step: Unmould and decorate. To remove the cake, run a warm, thin knife around the edges or gently warm the sides of the tin to loosen. Decorate the neapolitan mousse cake just before serving with fresh fruits, whipped cream, or chocolate decorations to enhance the presentation.