Vegan chocolate party traybake

Name: Vegan chocolate party traybake
Prep time: 30 min
Cook time: 45 min
Total time: 1 h 15 min
Cuisine: British, European
Ingredients: 215ml sunflower oil plus extra for the tin; 300ml soy milk almond or coconut milk; 2 tsp apple cider vinegar; 335g light muscovado sugar; 1 tsp vanilla extract or vanilla bean paste; 175g soy yogurt or coconut yogurt; 300g self-raising flour; 105g cocoa powder; 1 tsp baking powder; 1 tsp bicarbonate of soda; 100g dark dairy-free chocolate; 140g vegan spread at room temperature; 1 tsp vanilla bean paste or extract; 265g icing sugar sifted; ½–1 tbsp plant milk (whatever you used in the sponge); colourful sprinkles (make sure they're suitable for vegans), to decorate (optional)