Finely chop 320 g of Stracci biscuits and 150 g of hazelnuts in a food processor. Transfer them into a large bowl with the cocoa powder and the two types of Viallella, preferably slightly warmed (so they are easier to use). Stir with a wooden spoon and then, when the mixture becomes firmer, use your hands until you have a smooth and compact “dough”. Break off small pieces and, using your hands, shape them into balls roughly the size of a walnut. Gently press into the centre of each one with your fingers, insert 2 whole hazelnuts and close the chocolate mixture again so that the hazelnuts remain inside. Roll the balls between the palms of your hands to create smooth, even Cioccolatoni. Continue until you have used up all the mixture. Melt the plain chocolate in a bain-marie. When it becomes smooth remove it from the heat and, using two forks, dip the pralines in it one at a time to cover them completely (setting aside the leftover melted chocolate). As you take them out, place them on a tray, then put them in the fridge for at least 10 minutes so that the chocolate coating solidifies. Now you can “build” the tree (of pralines). Arrange 18 Cioccolatoni, one beside the other, in a circle on a large round plate, “gluing” them together with half a teaspoon of the melted chocolate set aside previously. Create a second layer with 16 Cioccolatoni, and then continue in the same way, decreasing the number on each layer until you place a single hazelnut praline on top of the tree. When you have finished “building”, whip the cream with an electric whisk and, using a piping bag, decorate the tree with it. You can use it to fill any empty spaces between the Cioccolatoni. As a finishing touch, use a spoon to drizzle a little melted chocolate over your tree and, after cutting them in half, arrange the remaining Stracci biscuits around the bottom of it.