Place the dates in a saucepan and cover with boiling water over a low heat.
Leave to simmer for 15 minutes or until the dates are soft. Drain and transfer to a blender and blend until the dates are of a smooth consistency.
In a large mixing bowl, whisk the butter and sugar, then one by one add the eggs. Mix in the date mixture. Sift together the flour and bicarbonate of soda, and fold in, mixing well.
Grease the baking tin and line with greaseproof paper. Pour in the mixture and bake for 60 minutes at 165C/330F/Gas 2.
While the sponge is baking, make the sauce. Pour half the cream, brown sugar and butter into a heavy-bottomed saucepan and bring to the boil, stirring frequently.
When the sauce reaches a golden colour, stir in the remaining cream and half the hazelnuts.
When the sponge is cooked, remove from the baking tin, onto a serving dish.
Pour over the hot toffee and hazelnut sauce and sprinkle the remaining hazelnuts on top. Serve with pouring cream.