Easy vegan square sponge cake similar to the popular UK school dinner cake.
Preheat your oven to 180°C/350°F (160°C/325°F if using fan or convection oven) and line an 8 x 10 inch rectangular baking tin.
In a small cup or bowl, mix the soy milk and apple cider vinegar together. Set it aside for a few minutes. This will act as vegan buttermilk in this recipe.
In a large bowl, mix together the flour, baking soda and caster sugar.
Add the vegan buttermilk, the oil and the vanilla extract to the bowl. Mix well until smooth, making sure there’s no dry mixture stuck to the bottom of the bowl.
Transfer the cake batter into your tin and bake for around 22-25 minutes. You can check that it’s cooked all the way through by inserting a toothpick through the center. If it comes out clean, the cake is ready.
Allow the cake to cool in the tin for around 10 minutes, then gently flip it out onto a wire rack to cool completely.
When the cake is completely cold, prepare your icing by mixing the powdered sugar and water together. It should be white and thick but slightly runny and spreadable. Add more powdered sugar for a thicker icing or more water for a thinner one.
Spoon the icing onto the cake and use a palette knife or the back of a spoon to spread it out evenly.
Decorate with your sprinkles of choice and leave the cake to stand at room temperature for 60 minutes. This will allow the icing to set properly and prevent it from sticking to your knife as you cut it.
Cut into square slices and enjoy alone or with warm vegan custard.