The chocolate-hazelnut paste popularised by the chocolate-makers of Turin in the 19th century was the forerunner of Nutella, the spread that children all over the world know and love. The dark chocolate-hazelnut ganache that fills these rich chocolate shortbread biscuits is an adult version of that childhood treat. Make these for a special tea party – they can be baked and assembled a day or so in advance.
Make the dough first. Put the softened butter into a mixing bowl, or the bowl of a freestanding electric mixer. Beat with a wooden spoon or a hand-held electric whisk, or the whisk attachment of the mixer, until very creamy. Add the sugar and beat until the mixture is much lighter in colour and texture. Scrape down the side of the bowl.
Sift the flour, cocoa powder and salt into the bowl and mix in with a wooden spoon or plastic spatula, then use your hands to bring the mixture together into a firm dough.
Turn the dough out onto an unfloured worktop (flour will leave white marks on the dough) and gently knead to a flat disc. (In very warm weather, or if your dough feels soft, wrap it in clingfilm and chill for about 15 minutes until firm enough to roll out.) Place the disc between 2 large sheets of clingfilm and roll out to 5mm thickness. Peel off the top layer of clingfilm and stamp out stars with the cutter. Gather up the trimmings, then re-roll and stamp out more stars. You want an even number – the dough should make 40 stars – but a few extra will allow for breakages.
Set the stars slightly apart on the baking sheets to allow for expansion, then cover lightly with clingfilm and chill for 15 minutes. Meanwhile, heat your oven to 180°C/160°C fan/350°F/Gas 4.
Uncover the stars and bake in the heated oven for about 12 minutes until just firm but not coloured – watch them carefully as the chocolate dough can quickly turn very dark around the tips (this would make the biscuits taste bitter). Remove from the oven and leave the stars to firm up on the baking sheets for 3 minutes before carefully transferring them to a wire rack to cool – the biscuits will be very fragile until completely cold. Leave the oven on.
While the biscuits are cooling, make the ganache filling. Tip the hazelnuts into a small baking dish or tin and toast in the oven for 7–10 minutes until a good golden brown. Remove 20 of the best-looking nuts and set aside for the decoration. Transfer the rest of the nuts to a food processor (there’s no need to wait until they are cold). Add the sugar and process until ground to a fairly fine and slightly sticky powder.
Break or chop up the chocolate into even-sized pieces and put into a small, heavy-based pan with the cream, butter and salt. Set over a very low heat and stir gently with a wooden spoon until melted and smooth. Remove from the heat and stir in the ground hazelnuts. Leave the ganache on the worktop, stirring frequently, until firm enough to spread.
When you are ready to assemble the stars, give the ganache a good stir. Using about a rounded teaspoon of ganache for each, gently spread the ganache over the underside of 20 stars. The biscuits are quite fragile – a tip might break off, but you can ‘glue’ it back with a dab of ganache. Top each star with a second star, placing the biscuits underside to underside and matching up the points.
To finish the stars, gently melt the chocolate. One at a time, dip the rounded base of each whole hazelnut in the melted chocolate, then set it in the centre of a star biscuit. Spoon the rest of the melted chocolate into the piping bag and snip off the end to make a small opening. Quickly pipe a fine zigzag of chocolate across each star. Leave until set, then very lightly dust with icing sugar. Store in an airtight container and eat within 5 days.