Melt the chocolate: Gently melt the dark chocolate using a double boiler or in the microwave in short bursts, stirring between each until smooth and glossy.

Make the raspberry puree: In a bowl, blend the fresh raspberries with lemon juice and sugar until slightly chunky. Strain to remove seeds if desired, then set aside.

Whip the cream: In a separate bowl, whip the heavy cream until soft peaks form.

Beat the egg whites: In another bowl, beat the egg whites with a pinch of sugar until stiff peaks form.

Combine the chocolate and cream: Gently fold the melted chocolate into the whipped cream with a spatula until smooth.

Incorporate the egg whites: Carefully fold the egg whites into the chocolate mixture to keep the mousse light and airy.

Assemble: Layer the chocolate mousse and raspberry puree alternately in clear cups or dessert glasses until full.

Chill: Refrigerate for at least 2 hours until set.

Serve: Just before serving, garnish with fresh raspberries or shaved chocolate for an elegant finish.