Method

• 1.
  In a stand mixer fitted with the paddle attachment (or using a medium bowl and an electric handheld mixer, or a food processor), beat the butter, white sugar, brown sugar and vanilla on medium-high speed for 3-5 minutes until the mixture is super light and fluffy. Using a spatula, scrape down the side of the bowl and, with the mixer on low speed, slowly add the flour and beat just to blend.
• 2.
  Divide the dough into four even pieces, placing each on a large piece of plastic wrap or baking paper. Fold the plastic or paper over so that it covers the dough, to stop your hands getting sticky. Using your hands, just like you’re playing with clay, form each piece of dough into a log shape, about 3cm in diameter. (I like these shortbreads thicker and smaller in diameter – like little fat, buttery coins.) Rolling the dough on the counter will help you smooth it out, but don’t worry about getting it totally perfect. Refrigerate for about 2 hours until totally firm.
• 3.
  Preheat oven to 180°C/160°C fan-forced. Line a baking tray or two with baking paper. Beat the egg with 1 tsp water for your eggwash. Brush the outside of the logs with the eggwash and roll them in more white sugar (for those really delicious crispy edges). Slice each log into 1cm-thick rounds, and place on the baking trays about 2.5cm apart. Sprinkle with sea salt flakes.
• 4.
  Bake for 12-15 minutes, until edges are just beginning to brown and shortbreads look sparkly and gorgeously golden. Let cool completely on trays before eating them all. (A warm shortbread is great, but a room-temperature shortbread is better.)