If using frozen raspberries, begin by letting them thaw completely, then discard any released liquid.

Blend the raspberries until smooth using an immersion blender or stand blender. Pass the purée through a fine sieve to remove the seeds, repeating if necessary until smooth, then discard the seeds.

Optionally, reserve about a quarter of the puréed raspberries and keep it aside for finishing the mousse later.

Place the remaining raspberry purée in a saucepan with the lemon juice and vanilla paste. Warm gently over low heat until it just begins to simmer.

While it heats, mix the gelatine powder with cold water in a small bowl until fully dissolved, then set aside.

Once the raspberry mixture is hot, remove it from the heat and whisk in the prepared gelatine until fully dissolved. Transfer the mixture into a large bowl and allow it to cool until it reaches room temperature.

Separate the eggs, keeping the yolks for another use. Place the egg whites and salt in a mixing bowl.

Whisk on medium to medium high speed until the whites become foamy and pale, reaching a soft shaving cream like texture. Gradually add the caster sugar, one spoon at a time, while continuing to whisk. Once all the sugar is incorporated, increase the speed to high and whisk until stiff peaks form.

Check that the raspberry mixture has cooled properly. Add about one third of the whipped egg whites and fold gently to loosen the mixture.

Pour the raspberry mixture into the remaining egg whites and fold carefully until fully combined and light in texture, taking care not to deflate the mixture.

Spoon the mousse into serving cups and let it set until firm.

When ready to serve, top with the reserved raspberry purée if you kept some, fresh raspberries, and optional dark chocolate shavings.