Sift the flour, salt and baking powder into a large bowl.
Add the chocolate chips and stir through.
In a second bowl, crack in the eggs and sugar and whisk for several minutes until very thick and pale. If you lift the whisk and drip ribbons across the top, they should take several seconds to sink back into the mixture.
Spoon about half of the egg mixture into the bowl with the flour, add the vanilla and fold through.
Repeat with half of the remaining egg mixture and mix again.
Add the remaining mixture until you have a soft but not sticky dough - it’s fine if you have some egg mixture left over.
Gather the dough up and divide into four.
Take each piece of dough, place on a lightly floured surface and roll out into a sausage about 3 cm (1¼ inches) wide.
Place the four sausages on the baking sheet, well spaced and use the flat of your hand to flatten slightly.
Bake
Bake in the oven for 25-30 minutes, until just slightly golden.
Remove from the oven, and turn it down to 160C/320F (140C/280F fan).
Place the tray on a wire rack and allow the biscotti to cool for a few minutes until safe enough to handle and firm up enough to slice.
Place the biscotti on a board and use a serrated knife to cut each sausage into 1cm (½ inch) slices.
Place the slices on an oven-safe wire rack and return to the oven for 15-20 minutes approx until golden and dried out.