Preheat the oven to 180C/350F/Gas4. Lightly oil a deep, 24cm/9½in springform cake tin with the olive oil.
Tip the crushed amaretti biscuits into the tin and swirl around so the sides and base are coated with a fine dusting. Turn the tin upside down over a large mixing bowl to catch excess amaretti.
Place the hazelnuts and chocolate in a food processor and, using the pulse button, process on and off until you have a grainy consistency but not a fine paste. Tip this into the amaretti bowl and add the sugar, brandy, orange zest and juice and pine nuts. Stir to combine, then add the egg yolks, stirring well. Sift over the flour, folding in gently.
Whisk the egg whites with a pinch of salt until stiff but not dry and gently fold this into the nutty mixture. Tip the mixture into the prepared tin and bake for about one hour or until a wooden skewer inserted into the middle comes out clean.
Remove to a wire rack and loosen off the sides of the tin. Allow to cool for an hour or so before carefully removing the base.
Eat at room temperature or slightly warm (so the chocolate melts a little).
