Cream the butter, sugar and lemon zest together until pale and fluffy. You can do this with a wooden spoon in a bowl or in a food mixer – but not in a food processor.
Now add the eggs by degrees to the mix, beating well in between each addition. I always add a teaspoon of the measured flour to the mix if it starts to look a bit curdled – this helps stabilise the mix.
With a large spoon (not the beater in the food mixer) gently fold in the flour, baking powder and hazelnuts. Fold carefully until well incorporated.
Divide the mixture between each cake tin and place into the oven. Cook for 20-25 minutes or until golden brown, springy to the touch and coming away from the edges. The last bit of the cake to cook is the middle, so make sure this bit is springy. You can also test by inserting a skewer – if it comes out clean, the cake is cooked.
Leave the cakes to cool for 15 minutes then turn out onto a cooling rack. When completely cold, fill with the lemon curd and icing (method below) and then dust the top with icing sugar.
For the icing:
Using an electric mixer (you can do by hand, but the mixer makes this frosting extra light) beat the butter and cream cheese together until thick and creamy. Gradually beat in the icing sugar and then the lemon juice. Leave in the fridge to set for 20 minutes before spreading three tablespoons into the middle of the sponge.