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.
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
All temperatures stated are for conventional ovens. Decrease the temperature by 20°C/25°F or gas mark 1 for "fan" or air-fryers.
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Notes
If using nonpareils or hundreds and thousands, the cookie dough must be mixed carefully, and not overmixed, this will prevent the mixing and bleeding of colors through the cookie dough.These cookies do not need to be rolled in sugar and sprinkles, but I like the extra crunch it gives the cookies.Resting the cookie dough in the fridge allows all the flour to hydrate and also solidifies the butter which helps the cookies maintain their shape during baking.