In a large mixing bowl combine the flour, baking powder, baking soda and salt. Then place to one side.
In a good size microwave safe jug, melt the butter on medium heat for around 60 seconds. Add the sugars, egg and egg yolk and vanilla extract into the jug along with the melted butter and mix everything together.
Pour this into the mixing bowl with the flour and fold everything together. Add in the white chocolate chunks, smarties and sprinkles and gently fold them into the cookie dough.
Place the dough into the fridge and leave for at least 2 hours or up to 3 days.
Once you are ready to bake your cookies preheat your oven to 180C/350F/Gas mark 4.
Take the cookie dough out of the fridge and allow it to sit at room temp for 10 minutes, this will make it a little easier to scoop.
I used an ice cream scoop to measure out my cookie dough, using a scoop helps to keep your cookies more uniformed in shape and size. Using an ice cream scoop gives you nice big bakery style cookies.
Scoop your cookies out and place them on a baking sheet with a little bit of space between each of the cookies as they do spread in the oven. I found 6 to a tray was perfect.
Bake your cookies for 10-12 minutes, but no more, the secret to bakery soft cookies is to underbake them by just a touch.
Once baked, take the cookies out of the oven and transfer them to a wire rack to fully cool down. If you want your cookies a little flatter then this is the perfect time to do that, just gently press down on them with the back of a metal spoon. I like my cookies thick but some like them a little thinner.
Once cool these cookies can be kept up to 5 days in an airtight tin or frozen for 2 months.