A 3-layer vegan sponge cake decorated with rainbow frosting.
Preheat your oven to 180°C/350°F (160°C/325°F for fan or convection ovens). Grease, flour and line three 7-inch cake tins.
Mix together the soy milk and apple cider vinegar and set it aside for 5 minutes. This will act as your vegan buttermilk.
In a large bowl, mix together the flour, baking soda and sugar.
Melt the vegan margarine and let it cool for a few minutes. Then add it to your bowl, along with the buttermilk you made earlier and the vanilla extract. Mix well and make sure there is no dry mixture stuck at the bottom of the bowl.
Divide the cake batter equally between the 3 cake tins. I like to use a scale to make sure they all have the exact same amount in them.
Bake the cakes for 18-20 minutes until a toothpick through the center comes out clean. Leave them in the tins for 10 minutes, then flip them out onto a wire rack and let them cool completley.
In a large bowl, whisk the shortening and vegan margarine until pale, smooth and creamy.
Keep whisking (I used a hand mixer) and start adding in the icing sugar a little bit at a time.
Once all of the icing sugar is incorporated, add in the vanilla extract and whisk one last time until you have a pale, fluffy buttercream.
Divide the buttercream equally between 5 small bowls and add a different colour of food gel to each one.
Transfer each colour of frosting into its own piping bag and cut the ends of to make a medium-sized hole for piping.
Place your first cake layer onto a cake stand and pipe a small circle of one colour of frosting into the center.
Using the next colour, pipe a larger circle around the edges of the first one and repeat until all of the colours are used. I recommend leaving around ½ an inch of space at the edges so the frosting doesn't spill out.
Place the second cake layer on top and repeat. Then top with the third cake layer and place the whole thing in the fridge while you prepare the white frosting. Keep the leftover coloured frosting in the piping bags for later.
In another large bowl, whisk the shortening until smooth. Start adding in the icing sugar a little at a time and keep whisking until it's all incorporated.
Add in the vanilla extract and a few tablespoons of the plant-based milk. Whisk for a few minutes until the frosting becomes super light and airy and increases in volume. If you'd like it lighter, just add in more plant-based milk until you get your desired consistency.
Remove the cake from the fridge and use a palette knife to spread the white frosting on top and down the edges. Use the back of a teaspoon to create swirly patterns on the frosting.
Return the cake to the fridge.
Cut open a disposable piping bag and lay it out flat. Alternatively, you can use plastic wrap or baking paper.
Using the leftover coloured frosting, pipe a line of each colour (side by side) onto the flattened piping bag. Then fold it up and place it inside of another piping bag with a large star nozzle.
Pipe small swirls around the top of the cake, fill the empty space with your colourful sweets and enjoy!