To start, flick the oven onto 180°C bake and line a 20cm square baking tin with baking paper.

Making the cookie base first, cream together the butter and sugars until pale and fluffy using either a free-standing mixer or  a hand-held beater.

Beat in the egg and vanilla, then on a low speed, mix in the flour and salt until the dough comes together.

Fold through the chocolate chips, then firmly press dough into the prepared baking tin.

Bake the base for 20 minutes, then remove from the oven and set aside. However while the base is baking in the oven, start on the cake mixture.

To make the cake, again cream butter and sugar until pale and fluffy.

Add vanilla and sour cream and beat until incorporated.

On a low speed, add one egg white at a time until all egg whites have been added and mixture is nice and silky.

In a separate bowl, sift in the flour and baking powder and set aside. Also measure out milk and water in a jug and set aside.

On a low speed, alternate with adding the flour and milk over 3-4 additions, beating between each. Once all of the flour and milk has been incorporated and mixture is smooth, gently fold through the sprinkles with a spatula. Make sure not to over mix.

Pour the cake mixture over top of the cookie base (the base should still be hot) and return to the oven and bake for another 40-45 minutes or until the toothpick comes out clean.

Set the cake aside and allow to cool completely before icing.

To make the buttercream, add butter to a mixer and beat for 5 minutes until pale.

Scrape down the sides and sift in half the measure of icing sugar and beat for another 3 minutes. Add final measure for icing sugar and beat for another 3 minutes.

Scrape down the sides once more and on a slow speed add the milk. Turn the speed back up and blast for another minute or so.

Smoother buttercream over top of cake and then cover with plenty more sprinkles. Slice and serve!