Preheat oven to 160°C (140°C fan forced). Grease 3 x 20cm round cake pans with cooking spray. Line the bases with baking paper.
Beat cake mixes, eggs, butter and milk in a large bowl with electric mixer on low speed for 1 minute, then on high speed for 3 minutes. Divide mixture between cake pans.*
Bake for 50 – 55 minutes or until a skewer inserted in the centre comes out clean. Cool for 10 minutes in pan, and then remove from pan onto a wire rack to cool completely. Trim cake tops until evenly flat and slice each cake in half horizontally.
Place the base half of a cake onto your cake stand and spread with a layer of frosting. Repeat with remaining cake halves. Spread frosting over top and sides of cake to crumb coat. There will be crumbs visible, these will be covered later. Refrigerate for 30 minutes until frosting is firm.
Empty frosting from 2 tubs into a mixing bowl. Colour with blue colouring to replicate your favourite shade of blue sky. Spread an even layer over the top and sides of the cake, and rotate the cake while spreading to achieve a smooth finish.
Place half the marshmallows onto the cake to replicate clouds. Place the remaining marshmallows in a microwave safe bowl with 1 tablespoon water and heat on high for 1 minute. Stir until smooth. If any lumps remain, return to microwave to heat in bursts of 10 seconds until marshmallows have formed a smooth liquid. Allow to cool slightly. Using a spoon, pour liquid marshmallow around the edges of the cake, allowing it to drip down the sides in an irregular pattern.**
Finish the cake with wisps of the Persian fairy floss just before serving, as it dissolves on standing.
*Cakes can be prepared individually as per pack directions. Butter and milk will provide a flatter finish; oil and water causes doming.
**Test for correct pouring temperature by pouring a little of the mixture at regular intervals while cooling. It can change quickly from being too thin, which looks patchy, to too thick to drip.