Combine the dates, butter, bicarb and boiling water. Stir until the butter has melted. Set aside to cool.
Preheat the oven to 180 C / Gas 4. Grease an 18cm (7 in) round deep cake tin and line the bottom with baking parchment.
Beat together the light brown soft sugar and eggs in a large bowl. Add the cooled date mixture, then sift in the flour, baking powder, cinnamon, nutmeg and salt. Add the hazelnuts and stir together until thoroughly mixed.
Pour the mixture into the prepared tin and level the top. Bake for 1 to 1 1/4 hours or until the cake is risen and nicely browned and a skewer inserted in the centre comes out clean.
Turn out onto a wire rack and leave to cool before icing.
For the icing: Combine the butter, lemon juice, vanilla, and icing sugar and beat until light and fluffy. Add more icing sugar or lemon juice to adjust the consistency as needed. Spread over cooled cake.
The cake can be kept, wrapped in foil or stored in an airtight container, for up to 5 days.
How to ice a cake video
Watch our How to ice a cake video and see how to ice cakes with ease. You'll be icing perfect birthday and special occasion cakes in no time!