Preheat your oven to 350°F (175°C).

Crush the digestive biscuits into fine crumbs using a zip-lock bag and a rolling pin or a food processor.

In a mixing bowl, combine the cookie crumbs, melted butter, and sugar until well mixed.

Press the mixture into the bottom of a silicone dome mold to create an even layer. Bake for 10-12 minutes until slightly golden. Let it cool.

While the base cools, melt the chopped dark chocolate in a heat-proof bowl over a pot of simmering water, stirring until smooth. Remove from heat and let cool slightly.

In a separate bowl, whisk the egg yolks and sugar until thick and pale, then mix in the lemon juice and zest.

Stir in the melted chocolate until fully incorporated.

In another bowl, whip the heavy cream to soft peaks. Gently fold the whipped cream into the chocolate mixture.

In a clean bowl, whip the egg whites until stiff peaks form and carefully fold them into the mousse mixture.

Spoon the chocolate mousse over the cooled cookie crunch base in the dome mold until filled. Smooth out the top with a spatula.

Cover with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.

Once set, carefully remove the mousse dome from the mold. Place it on a serving platter and decorate with whipped cream, chocolate shavings, and lemon zest.