For The Vegan Cake For The Vegan Buttercream

1. Get your oven to 180°C/160°C fan/gas 4. You'll need to grease three 20cm round cake tins, then cut and fit baking parchment into the bases and sides of the tins. If you don't have three tins, it's okay - just cook the batter in batches. Then take a jug and whisk the milk and vinegar in it — the milk should slightly curdle.
2. Go ahead and whisk together the sugar, oil, and vanilla extract in a large bowl. Now, whisk in the yogurt, and make sure to break down any stubborn sugar lumps. Pour in the soured milk and make sure to mix well.
3. In a separate bowl, sift the flour, cocoa powder, baking powder, bicarbonate of soda, and ¼ tsp salt. Stir well to combine everything. Now, gradually whisk the wet ingredients into the dry ingredients until you have a smooth batter. Just be careful not to over-mix it.
4. Now, divide the batter evenly between the tins and bake for 25-30 mins, or until they're well-risen, springy, and when you insert a skewer into the center, it comes out almost clean. A few sticky crumbs are fine, but the mixture shouldn't be wet.
5. Let the cakes cool in their tins for 20 mins. Then, carefully turn them out onto a wire rack so they can cool completely.
6. For the vegan buttercream, you'll need to melt the chocolate in the microwave or in a bowl set over a pan of simmering water. Once it's melted, let it cool. In a stand mixer bowl, beat the spread and vanilla on high speed for a few mins until it's pale and fluffy. Gradually, add the icing sugar. Start beating on slow speed, then increase to max until the mixture is light and creamy. Pour in the cooled chocolate and combine thoroughly. Chill the buttercream for at least 30 mins before using it.
7. Let's assemble the masterpiece! Start by leveling the tops of the sponges with a sharp knife. Place one of the sponges on a serving plate, cake stand, or a 20cm round cake drum. (Using a cake drum makes it easier to ice the cake neatly and move it onto a stand or plate later.) Spread a layer of buttercream over the sponge, using a palette knife for an even, neat finish. Top with the second sponge and spread with another layer of buttercream.
8. Finally, place the last sponge upside-down on top, so the bottom of the cake becomes the top. This helps keep the icing neat and crumb-free. Spread the sides of the cake with buttercream. Steady the top sponge with a hand if needed. Once the sides are covered, spread a thin layer of buttercream on top. Use your palette knife to neaten the top and sides. If you have a side scraper, use it to sweep around the sides and top to give the coating a clean finish. (This is called a crumb coat, trapping crumbs for a neat, firm base.) Now, chill the cake in the fridge for 1-2 hours to firm up.
9. To finish, cover the sides and top of the cake with the remaining buttercream, and press the sprinkles against the bottom of the cake, about a quarter to a third of the way up. Decorate the top as you like. You can use a circle of sprinkles or, for a fancier look, pipe little swirls around the top edge of the cake with any remaining buttercream scraped into a piping bag fitted with a large open star nozzle, and finish with even more sprinkles!
10. Keep the cake in the fridge to stay firm. Remove 1 hour before serving.