Crush cookies or nuts and press them into the bottom of a lined tray to form the base. Chill in the fridge.
Gently melt the dark chocolate using a double boiling method or microwave, stirring until smooth.
In a chilled bowl, whip the heavy cream with powdered sugar, vanilla extract and salt until soft peaks form.
Let the melted chocolate cool slightly, then gently fold it into the whipped cream until fully combined and smooth.
Spread the mousse mixture evenly over the cookie/nut base.
Refrigerate for at least 4 hours or until firm enough to cut into the bars.
Cut into bars, garnish with shaved chocolates or nuts if desired and enjoy.