Cream the room-temperature butter and sugar together using an electric hand mixer or stand mixer for 2-3 minutes. The butter should be light, fluffy, and lightened in color.
113 grams butter, 150 grams white granulated sugar
Add the vanilla extract and room-temperature egg and beat for 2-3 minutes until the egg is completely incorporated and the mixture is nice and fluffy.
1 egg, 2 teaspoons vanilla extract
In a smaller bowl, combine the flour, baking soda, and salt. Whisk together and then add to the butter-sugar-egg mixture. Toss in the sprinkles and mix to form a soft cookie dough.
180 grams plain flour, ¼ teaspoon table salt, ½ teaspoon baking soda, 80 grams sprinkles
Using an ice-cream scoop or 2-tablespoon cookie scoop and portion out the cookies. Place the portioned cookies on a sheet of parchment on a plate or baking tray. Allow the cookies to rest in the fridge for 2 hours or overnight. If resting for longer than 2 hours, store the cookies in an airtight container in a single layer or cover the plate with plastic wrap.
Preheat the oven to 180°C/355F or 160°C fan. Line two baking trays with baking paper.
Place 2-4 tablespoons of granulated sugar and 2 tablespoons of sprinkles in a small bowl. Dip the tops of the cookies all the way to the base into the mixture and place them on the baking trays about 7cm or 3 inches apart. Bake for 15 to 20 minutes. Allow cooling on the baking trays for 5 minutes before transferring them to a cooling rack.
4 tablespoons white granulated sugar, 2 tablespoons sprinkles