Preheat your oven to 180c fan and line a brownie tray with baking paper.
To create your vegan buttermilk, combine the apple cider vinegar and soy milk and mix well. Set these aside for 5 minute to allow the mixture to thicken and curdle.
In a large mixing bowl, add the sugar and olive oil. Use a wooden spoon to cream these together until pale and fluffy.
Whisk in the buttermilk mixture and vanilla extract, then sift in the dry ingredients. Fold the wet and dry ingredients together to create a smooth, airy batter.
Gently stir in the mixed berries, then transfer your mixture to your baking tin. Bake for 30-35 minutes until cooked through. Your cake is done when you can insert a toothpick and this comes out clean.
Allow your cake to cool completely on a wire rack. Meanwhile, for the topping, whip your cream to soft peaks, then add in the icing sugar and a generous dose of vanilla extract or powder.
Transfer your cream to a piping bag and decorate your cake with some gorgeous piped swirls. Top with fresh fruit and serve.