Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Grease and line 3 x 18cm (7inch) round tins. In a mixing bowl, beat half of the white vegetable fat with half of the sugar until creamy in texture.
Gradually whisk in 2 eggs and ¼ tsp salt. Sift half the flour and 2 tsp Baking Powder on top. Using a large metal spoon, fold in the dry ingredients. Stir in half the milk to soften the mixture to make a soft dropping consistency.
Divide the mixture equally between 3 bowls. Choose 3 different Gel Food Colours and mix in sufficient drops of one colour into each bowl of mixture to form a vibrant shade (approx. 8g of each colour).
Spoon the mixture into the cake tins, smooth the tops and bake in the oven for about 15 minutes until lightly golden and just firm to the touch. Leave to cool for 5 minutes then turn on to wire racks to cool.
Using the remaining ingredients and repeat steps 1 and 4 with the remaining 3 colours.
Using a palette knife spread the Vanilla Buttercream Style Icing lightly over 5 of the cake layers. On a serving plate, stack the layers on top of each other.
To cover the cake start by adding a crumb coat to your cake. Add a thin layer of buttercream using a pallet knife, starting from the top and working your way down the sides of the cake. Scrape any excess buttercream off using your pallet knife so you have a smooth thin layer covering your cake. Chill in the fridge for 30 minutes. Once chilled add the remaining buttercream again working from the top and working your way down the sides of your cake. Use your pallet knife to gently scrape any excess buttercream to give you a smooth finish.
Store your cake in an airtight container or a tall cake box until you are ready to decorate your cake.
Divide the block of Fondant into 6. Take one piece of Fondant, knead to soften and then add a few drops of Red Colour Gel. Knead the Food Colour Gel into the fondant with your hands and using a bit of icing sugar if it gets too sticky. Repeat adding a few more drops of Red Colour Gel until you have a bright vibrant colour.
Repeat step 1, adding a different Colour Gel until you have 6 different coloured balls of fondant.
Then divide each coloured ball of fondant into two, wrapping one half in cling film to stop it drying out and putting to one side.
Smear some trex onto a clean work surface to stop the fondant sticking then take each unwrapped ball and roll it into a sausage about 1/2cm thick and 20cm long.
Take a small bottle or pot such as a bottle of Dr Oetker vanilla extract to use as a guide, and place on a tray lined with non- stick baking paper.
Curve each coloured sausage over the bottle in the shape of a rainbow starting with purple, then on top of that blue, then green, then yellow, then orange, then finally red. Once you have your rainbow shape take a knife and cut off the ends of the bottom edge of the rainbow in a straight line so the rainbow measures about 10cm high.
Take a little pot of clear alcohol such as vodka, or alternatively use cold water, and with a little brush glue each strand of the rainbow together by brushing the alcohol in between each colour, you only need a very small amount otherwise the fondant will become sticky.
Take two cocktail sticks and insert into the bottom of the yellow strand at both sides of the rainbow, this will help the rainbow stand on top of the cake.
Leave the rainbow uncovered to air dry and harden overnight
To make the fondant balls, take your reserved coloured balls of fondant and unwrap your first colour. Roll this out into a long sausage about 1cm thick. Cut the sausage up into 1cm wide pieces and roll each piece between your palms to make a ball. Place the balls on a tray lined with non- stick paper.
Repeat with each colour until you have balls in all your rainbow colours. Leave to air dry overnight to harden.
Once set carefully stick your rainbow into the centre of the top of the cake.
Add the fondant balls in alternate colours to the base of the cake by lightly pressing into the buttercream.
Finish by piping some vanilla buttercream into clouds at the base of each end of the rainbow. To pipe cut the end off the piping bag in a straight line to create a hole 1cm wide. If the icing peaks when piped, dip finger in a little cold water to dab down the peak to create a more rounded shape. Then decorate with Bright and Bold sprinkles.
To make your own buttercream mix 300g softened unsalted butter with 2 tsp Dr Oetker vanilla extract, 100ml double cream and 800g icing sugar. Blend well together with an electric mixer until the mixture is light in colour and fluffy in texture.