Preheat oven to 350°F (180°C). Line two 9-inch round pans with parchment paper. Grease the pans with coconut oil on the sides and bottom, parchment paper included. Set aside.
In a medium mixing bowl, combine apple cider vinegar, almond milk, canola oil, and vanilla extract. Set aside 10 minutes to curdle.
In another large mixing bowl, sift flour. Then, whisk in caster sugar, salt, and baking powder. Set aside.
Pour the dry ingredients into the wet, give a quick whisk to incorporate until ingredients are combined. Don't over-whisk, or the cake will be dense.
Stir in rainbow sprinkle to incorporate, again don't over mix, or the cake will be dense and gummy.
Divide the batter evenly in half to fill the two round pans.
Bake both pans on the center rack of your oven for 30-35 minutes or until they are golden brown and a pick inserted in the center comes out clean. You can also cook one in the center rack and one in the rack just under. Swap the pans halfway to make sure they bake at the same speed.
Cool down on a rack completely before adding any frosting.
Meanwhile, prepare your vegan buttercream frosting following my recipe here.
Place one layer of cake on a large serving plate, add half of the batch of frosting in the center of the cake and use a spatula to spread the frosting all around the cake. It's ok if it drips on the sides.
Top up with the second cake layer, add another batch of vegan frosting and repeat the same action, spreading frosting on sides as well until it entirely covers the top and sides of your cake.
Decorate with more rainbow sprinkles and place the cake for 1 hour in the fridge to set the frosting and blend flavors.
Store cake in an airtight cake box in the fridge for up to 4 days or freeze and thaw the day before serving.