Make the candied orange slices a day ahead if you can. Put the sugar and 300ml water in a medium saucepan and bring to the boil. Reduce to a gentle simmer, add the orange slices and cook for 1 hr-1 hr 10 mins or until the pith is translucent, turning occasionally. Line a baking tray with parchment. Carefully remove the slices from the syrup, place on the prepared tray and set aside to dry (at least 8 hrs) before cutting in half. Reserve the syrup.
Heat oven to 180C/160C fan/gas 4. Lightly grease a loaf tin (mine was 900g) with olive oil and line with a strip of baking parchment.
Grind the hazelnuts in a food processor until they resemble coarse breadcrumbs (do not blitz them too much or they will become oily), then add to a bowl along with the orange zest, a pinch of salt and the rest of the dry ingredients. Mix together until evenly combined.
Pour the oil and the orange juice into a jug and mix together. Put the eggs and sugar in a tabletop mixer or large bowl and whisk together for 5-10 mins or until the mixture has tripled in volume and holds a ribbon on the surface when the beaters are lifted out. Slowly pour the oil mixture into the egg mixture and fold together until combined.
Add the flour mixture to the egg mixture in 3 or 4 additions, folding together until combined. You can’t sieve this mixture over the eggs because of the hazelnuts, but try not to dump the flour in one place – you need to be careful and fold the batter to retain its lightness. Once fully combined, pour the batter into the prepared loaf tin and bake for 50-55 mins or until a skewer inserted into the centre of the cake comes out clean.
Prick the top of the cake all over, then pour over 5 tbsp of the reserved orange syrup. Cool in the tin for 15 mins, then turn out onto a wire rack to cool completely. Trim the top of the cake (keep this for a mini trifle) and turn out, cut-side down, onto a serving platter.
To decorate, pour the orange juice into a small pan and bring to a simmer. Put the chocolate in a small bowl, pour the orange juice over and stir together to form a smooth ganache. Set aside in the fridge until thickened, about 20-25 mins. Tip the hazelnuts into a small bowl and add the gold powder with a dash of water, stirring together to coat. Put the ganache in a piping bag fitted with a small round piping tip. Pipe in peaks over the top of the cake, decorating with the golden hazelnuts and the orange slices. Will keep for up to 3 days in an airtight container.