In a bowl, roughly crush the digestive biscuits and mix with the chopped hazelnuts. Toasting the nuts is optional, but if you would like to do so, place on a greased baking tray for 10-15 minutes in a 350 F (180 C) oven. Then rub the hazelnuts with a paper towel to remove the skins.
In a separate bowl, combine the butter, chocolate, heavy cream and Nutella. Microwave, stirring every 30 seconds for 2-3 minutes or melt over a double boiler until smooth.
Pour the chocolate mixture over the cookie/hazelnut mixture and stir gently to combine. Firmly press the cookie mixture into a 8 inch (20 centimeters) cake tin lined with foil and greased with butter.
Chill for 1-2 hours or until firm.
For the glaze, place the chocolate and chocolate hazelnut spread in a bowl.
Bring the heavy cream to a boil in the microwave/in a saucepan and pour the spread and chocolate.
Let the mixture sit for 30 seconds then stir till smooth.
Remove the cake from the cake pan and peel off the foil.
You can wait till the glaze hardens in order to spread it atop like a frosting, or you can pour it over the cake so that it forms a glossy outer coating.
Sprinkle the cake with toasted, chopped hazelnuts to garnish.