In a medium bowl, combine the crushed digestive biscuits with melted unsalted butter until the mixture resembles damp sand. Press this mixture into the bottom and up the sides of a 9-inch tart pan. Refrigerate for about 20 minutes to set up.

While the crust chills, beat the heavy whipping cream using an electric mixer until soft peaks form. Gradually incorporate the powdered sugar while mixing until stiff peaks are achieved in about 2-3 minutes.

In another bowl, blend the softened cream cheese and chocolate hazelnut spread until creamy and smooth. Add the pure vanilla extract and a pinch of flaky sea salt, mixing until well combined.

Carefully fold the whipped cream into the chocolate hazelnut mixture using a spatula. Start with a third of the whipped cream, then gently fold in the remaining whipped cream until uniform and airy.

Pour the chocolate filling into the chilled biscuit crust, smooth the surface evenly, cover with plastic wrap, and return to the refrigerator. Chill for at least 4 hours or overnight.

Once set, remove the tart from the refrigerator. Garnish with chopped hazelnuts and dark chocolate shavings, slice with a sharp knife, and serve chilled.