Preheat a fan forced oven to 170C / 350F. Spray the bottom of a 20cm x 20cm / 8 x 8-inch baking tin with oil spray and line with baking paper. Set aside.

Add both flours, salt and baking powder to a large mixing bowl. Use a whisk to combine. Set aside.

To a separate bowl add the melted and cooled butter and brown sugar and whisk until well combined. Add the egg, egg yolk and vanilla extract and whisk to combine.

Add the dry ingredients and sprinkles to the wet ingredients and use a spatula to combine until a dough forms.

Transfer the dough into the baking tin and spread evenly. Sprinkle with remining chocolate chips. Bake for 25 minutes. Lift out of the baking tin and cool completely on a cooling rack. Use a large knife to slice into 9 pieces.

Add your softened butter, cold cream cheese and vanilla extract to a large bowl. If using a stand mixer fit it with the paddle attachment. Beat on high speed for 3 minutes until pale and fluffy.

Scrape down the bowl and add all of the sifted powdered sugar and the pink food gel and begin beating on low speed. When no dry ingredients are showing, turn the speed up to high speed and beat for 5 minutes.

Take the cooled cookie bar out of the cake tin. Add the frosting to the cookie layer and spread around evenly using a spoon or spatula. Finish with a sprinkle of rainbow sprinkles.