Ingredients

Step by step

1. Preheat oven to 180°C, fan 160°C, gas 4. Grease and line a 20cm x 30cm traybake tin with baking paper.
2. Roast the hazelnuts on a baking tray in the oven for 8-10 minutes until richly golden brown. Leave to cool. Chop the chocolate and melt it in a heatproof bowl over a pan of barely simmering water. Remove from the pan and leave to cool.
3. Transfer 125g of the roasted hazelnuts to a food processor and blend until finely chopped. Add the flour, baking powder and a pinch of salt and process again until the nuts are very finely ground. Cut the remaining hazelnuts in half and set aside.
4. Cream the butter and sugar with an electric mixer for 5 minutes until very light and fluffy. Beat in the vanilla, followed by the eggs, one at a time, adding a heaped spoonful of the flour and nut mixture with the third and fourth egg. Gently fold in the remaining flour mixture, followed by the melted chocolate.
5. Spoon into the prepared tin and level the top. Bake for about 30 minutes until the sponge springs back when lightly pressed with your fingers, and a skewer pushed into the centre of the cake comes away cleanly. Remove from the oven and leave the cake to go cold in the tin, then unmould.
6. For the icing, melt the butter in a small pan over a medium heat. Add the muscovado sugar and cream and melt, stirring, until very smooth. Pour into a heatproof mixing bowl and beat in the icing sugar using a hand-held electric whisk to make a smooth and glossy icing.
7. Swiftly pour the icing onto the top of the cake and gently spread out in an even layer, swirling it with the tip of the knife as you do so. Scatter over the roasted nuts and the salt flakes then drizzle the melted chocolate across the cake. Leave to set then cut the traybake into squares.