Process the butter, both sugars, flour, cornflour, salt and half the hazelnuts to a crumbly texture in a food processor. Pat an even layer into the base of a greased 23cm/9in springform tin. Bake for 20 minutes, until golden brown. Remove from the oven and leave to cool.
Reduce the oven temperature to 150C/300F/Gas 2.
For the filling, bring the double and single creams to the boil in a saucepan, then remove from the heat and add the whiskey and coffee essence or coffee. Cool slightly before stirring in both the chopped chocolates. Stir and, when the chocolates have melted, stir in the eggs. Pour the filling over the base and bake on the centre shelf of the oven for about 20 minutes, until just set but still slightly wobbly at the centre. Remove from the oven and leave to cool completely.
To make the topping, soften the leaf gelatine (or dissolve the powdered gelatine, if using) in two tablespoons of water for a few minutes, then heat gently to dissolve. Set aside to cool slightly. Whip the cream until it forms soft peaks and sift in the icing sugar. Mix in the gelatine and Baileys Irish cream. Spread an even layer over the chocolate filling and leave to set.
To make the chocolate sauce, bring the milk and cream to the boil. Remove from the heat and leave to cool slightly. Add the chocolate and then stir until well mixed and all the chocolate has melted.
Serve the tart in slices, with some chocolate sauce drizzle over and around, lightly whipped cream on the side and the remaining hazelnuts scattered over.