In a medium-sized bowl sieve all the dry ingredients together. The cake mix, flour, cornflour, baking soda and salt and place to one side.
In your mixer or in a large mixing bowl using an electric hand mixer beat the butter and sugars together. Until they are well combined and light and fluffy. About 3 minutes.
Add in the egg and vanilla extract to the batter and mix for 1 minute.
Put your mixer on slow and add in the flour mixture and mix until the flour is just incorporated.
Pour in the sprinkles and give them a gentle mix into the batter. Cover your bowl tightly with cling film (plastic wrap) and pop it into the fridge for at least 2 hours.
Preheat your oven to 180C/350F/Gas mark 4.
Line your cookie sheet with parchment paper or line it with a silicone mat and place equal size balls of dough on it about 2 and 1/2 inches apart. I used a 1/8 cup for measuring my dough its about 2 tablespoons in size.
Pop them in the oven and bake them for 10 minutes. This will be the optimum time for soft underbaked cookies. If you like your cookies a little crunchier then bake them for an extra minute or two.
When you take them out of the oven allow them to sit on the hot cookie sheet for 5 minutes before moving them to a wire rack to fully cool down.
Kept in an airtight tin these cookies will be good for 5 days.