In a medium saucepan, melt the chocolate chips over low heat, stirring constantly until smooth. Remove from heat and let cool slightly.

In a large mixing bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the granulated sugar and continue beating until stiff peaks form.

In another bowl, whisk the egg yolks and vanilla extract into the melted chocolate until well combined.

Gently fold one-third of the egg whites into the chocolate mixture to lighten it. Then, carefully fold in the remaining egg whites until no white streaks remain.

In a separate bowl, whip the heavy cream until soft peaks form. Gently fold the whipped cream into the chocolate mixture until fully incorporated.

Spoon the mousse into individual serving cups or bowls and refrigerate for at least 2 hours, or until set.

Serve chilled, garnished with chocolate shavings or fresh berries if desired.