This classic vanilla cookie is great for the holiday time, since you can make the dough in advance and chill or freeze it until you want to bake it. It makes a great base cookie to ice, so keep a supply of dough on hand for last-minute cookie emergencies.
Beat the butter, icing sugar and granulated sugar well, using electric beaters or a stand mixer fitted with the paddle attachment, until smooth and fluffy. Add the egg, egg yolk and vanilla and beat in well. Add the flour and salt and mix until the dough comes together (it will be soft.) Shape the dough into 2 logs about 1 ½-inches (35 mm) across and wrap in plastic wrap or parchment and chill until firm, about 2 hours. Alternatively the dough can be made ahead and frozen, then thawed in the fridge before baking.
Preheat the oven to 350°F (180°C) and line 2 baking trays with parchment paper.
Brush each log with the egg white and roll it into decorator’s sugar of your choice. Slice the dough into cookies ¼-inch (6 mm) think and place them an inch (25 mm) apart on the prepared baking trays. Bake for 10-12 minutes until the cookies lift easily from the tray and are lightly browned on the bottom. Allow the cookies to cool on the trays on a cooling rack.
The cookies will keep in an airtight container for up to 4 days.