Line the base of a 20cm loose-bottomed, deep tart tin with baking parchment. To make the fudge sauce, put the chocolate, cream and golden syrup in a small pan and heat gently, stirring occasionally, until the chocolate has melted. Stir in the vanilla extract and icing sugar. Cool.
Heat the oven to 190C/fan 170C/gas 5. Rub the butter, flour, cocoa powder and dark brown sugar together until they resemble breadcrumbs. Press them into the base and sides of the tart tin. Cover with a piece of greaseproof paper and weigh down with baking beans. Bake for 8-10 minutes until just set. Remove the paper and baking beans and cool on a rack, still in the tin.
To build the pie, spread 1/2 the cooled fudge sauce over the base of the tart. Cut the coffee ice cream into slices and press them in to make the bottom layer of the pie, filling any gaps with ice cream. Cover with clingfilm and return to the freezer for at least 1 hour.
Scatter the hazelnuts over the coffee ice cream layer. Slice the chocolate ice cream and press the slices over, to make a second layer. Cover and return to the freezer for at least an hour.
Once the ice cream is completely firm, spread the rest of the fudge sauce over the top. Freeze until firm, then cover with clingfilm. Store for up to a week in the freezer.
An hour before serving, loosen around the edge of the pie with a thin knife. Put the pie on a tin can and push down the outside ring. With a palette knife, loosen the base and lift onto a serving plate. Put in the fridge to soften until ready to eat. Cut into slices and serve by itself or with whipped cream.