Heat the oven to 180C/160C fan/gas 4. In a large bowl, combine the flour, cocoa powder, mixed spice and 1 tsp salt. In another bowl, combine the olive oil, sugar, eggs, vanilla essence and grated courgette. Mix the dry and wet mixture until just combined, then fold in the toasted hazelnuts. Line a 24cm cake tin with greaseproof paper, then pour in your mixture. Bake for about 40-50 mins, or until a knife inserted into the middle comes out clean. Cool in the tin for 10 mins, then turn out onto a wire rack and leave to cool.