In a medium bowl, use an electric whisk on medium high speed to beat together the butter, granulated sugar, brown sugar, & vanilla until super light & fluffy, about 3–5 minutes.Use a spatula to scrape down the sides of bowl. With the mixer switched to low, slowly add the flour, followed by chocolate chunks, & beat until juuuust blended.
Divide dough in two & place each half on a large piece of cling film. Fold the cling film over the dough & use your hands/the counter to form the dough into a log shape. Repeat the process with the second half of dough. The logs should be about 2-inches in diameter. Place the logs into the fridge until firm, about 2 hours.
Preheat the oven to 180°C & line two baking sheets with baking parchment; set aside.
Taking one log at a time, remove the cling film & brush the outside of log with the beaten egg. Repeat with the second log. Once coated with egg wash, roll the logs in demerara sugar (I had mine set up in a shallow bowl/plate).
Slice each log into ½-inch thick cookies. Arrange on the prepared baking sheets about 2-inches apart & sprinkle with sea salt. Bake in the preheated oven until the edges are just beginning to brown, 12–15 minutes. Leave to cool on the tray for 5 minutes before moving to a wire rack to cool completely.
Enjoy while still warm in the middle for those gooey chocolate bites!