Heat oven to 180C/160C fan/gas 4 and line a large baking sheet with baking paper. Whisk together the eggs and sugar until light and fluffy. Sift the flour and bicarbonate of soda into a bowl, then add the orange zest, hazelnuts and chocolate chips. Fold into the egg mix to make a soft dough. Tip onto a lightly floured surface, shape into a 25cm sausage shape with lightly floured hands, then transfer to baking sheet. Flatten to a 3cm thickness, then bake for 30 mins until lightly browned on top.
Remove and slide onto a chopping board, then reduce the oven to 160C/140C fan/gas 3. Cut the log into 1cm thick slices and return to the baking sheet, cut-sides up. Bake for 10-15 mins until crisp, then cool on a wire rack. Will keep in a tin for 3 weeks.