Into a medium bowl, sift the flour baking powder and salt. Use a whisk to combine. Set aside.
Add the softened butter and both sugars into a large bowl and use an electric hand beater to cream until pale and fluffy, about 2-3 minutes.
Add the egg and egg yolk and beat until combined. Scrape down the bowl and add the dry ingredients and sprinkles and use a spatula to mix until a dough forms. Take care not to overwork otherwise you’ll end up with tough cookies.
Use a medium scoop (about 2 tbsp) to scoop out 15 cookie dough balls. Place on a large baking tray lined with baking paper and chill in the fridge for 2 hours, overnight for a truly delicious and fudgy cookie experience!
Preheat a fan-forced oven to 150C/300F (175C/350F with no fan). Space the cookies out onto two baking trays lined with baking paper. Bake one tray at a time in the middle rack for 10 minutes or until golden around the edges.
To make your cookies perfectly round, use a cookie cutter to shuffle around the soft cookies until they form a rounder shape. Allow to cool completely before serving.
Notes
Rainbow sprinkles - it’s important to use the right sprinkles here. The ones I used were American Style Rainbow Sprinkles. These are slightly shiny in appearance, bright in color and don’t bleed in bakes. If you’re in Australia, these are really hard to find, but you can use confetti sprinkles. These are different to 100’s and 1000’s.Sprinkles - I used the rainbow sprinkles known as ‘jimmies’. They’re a type of rainbow sprinkle that won’t bleed as they bake. If you’re in America, the Wilton ones are great. Aussies, I buy mine on Amazon. They’re the Hoosier Hill Farm Ones.Storage - cookies can be stored in an airtight container for up to 5 days.Nutrition - is based on per cookie.A note on measurements - all ingredients in this recipe are offered in Australian cup measurements and weight measurements. Weight measurements are created by me, in my kitchen here in Melbourne Australia. The best way to get recipe success is to measure ingredients by weight, using kitchen scales.