Add all your ingredients into a food processor ( or use a stand mixer): flour, sugar, salt, cubed room temperature butter and egg. Pulse until you get a nice malleable dough (it should be/feel like play dough), then add chocolate chunks and pulse for a few seconds just to incorporate (not for too long or it will melt).
Turn the dough out onto a floured work surface, divide it into 2 parts. Shape each into a 2 inches / 5 cm (in diameter) compact log. Wrap (tightly) with plastic wrap, let chill for 2 hours in the fridge.
Preheat the oven at 355° F / 180° C.
Place the logs on the floured work surface, slightly brush with beaten egg (you'll have leftovers) and evenly coat with Demerara sugar.
With a sharp knife (or serrated), slice the logs into ½ inch cookies. Place them on a lined baking sheet, about 1 inch apart (they won't spread that much)
Bake for 13-15 minutes, let them cool on a cooling rack, then sprinkle with flaky salt.