Melt 1 cup heavy cream, chocolate chips, vanilla and salt together in the top of a double boiler until chocolate is completely smooth.
Pour chocolate mixture into a bowl and cool to room temperature, stirring occasionally. *Speed up the cooling process by putting the bowl into the refrigerator.
In a large mixing bowl, beat 2 cups heavy cream with 1/4 cup sugar until stiff peaks begin to form. (This takes several minutes.)
Fold whipped cream mixture into the cooled chocolate mixture. Put into a large serving dish or spoon into individual serving bowls. Chill mousse in refrigerator for at least 6 hours before serving.
If desired, garnish with whipped cream and powdered cocoa (or chocolate shavings) immediately prior to serving.