Preheat your fan oven to 160˚C (325˚F) or 175˚C (350˚F) if you're not using a fan assisted oven. Line your cookie sheet with baking paper.
In a bowl of a stand mixer, fitted with a paddle attachment, beat together unsalted butter, sugar and orange zest until combined. You don't want to add any extra air or fluff, so beat it until just combined.
In a medium sized bowl sift together flour and salt, then add them to the butter-sugar mixture. Mix on low speed (or by hand) until it all starts coming together to form a dough. Don't overmix. It's fine if it's not homogenous.
Transfer the dough on a sheet of cling film, flatten to form a disk while wrapping it in cling film and put in the fridge to chill for 30-60 minutes.
Once the dough has chilled, roll it between two sheets of baking paper to 4-5 mm (¼ inch) thickness. Alternatively, you can roll the dough on a lightly floured surface. Using a 5 cm (2 inch) round cookie cutter, cut out the cookies and place them on a prepared baking tray. (You'll need two or do it in two batches).
Bake for 10-15 minutes, until the edges start to brown. Cookies will be fragile at this point, leave them on the tray for 2-3 minutes to cool a little bit, then transfer them to a cooling rack to cool completely.
For the dipping chocolate: melt 80 g in a microwave, in 30 sec bursts, or over a bain marie until just melted. Then add the rest of the chocolate chips, mix it together and leave at room temperature, stirring often until it's all combined and smooth. You want chocolate to be slightly cooler than body temperature and glossy.
Dip the cookies in chocolate and place them on a baking tray with parchment paper to cool. If you want to drizzle your cookies with chocolate, then put it in a small piping bag, snip the end of to a small round and drizzle away. :)
You can use a hand mixer, if you don't have a stand mixer.
If you still don’t own one, I advise you to buy a kitchen scale – they’re cheap and easy to use, and always guarantee the same results in baking! :)
If dipped chocolate doesn't harden at room temperature, put the cookies in the fridge for a little bit before eating. That happens when chocolate isn't tempered, which isn't easy to do.
You can make bigger cookies or use entirely different shape, it's up to you. Just remember you'll might have to prolong the baking time for bigger cookies.