Prepare Vegan Buttermilk: Combine 1 cup of dairy-free milk with 1 tablespoon of apple cider vinegar in a bowl. Stir gently and let it sit for about 5 minutes.

Mix Dry Ingredients: In a separate bowl, whisk together 2 cups of self-raising flour, 1 cup of caster sugar, 1 teaspoon of baking powder, and 1 teaspoon of bicarbonate of soda.

Combine Wet Ingredients: Mix the prepared vegan buttermilk with 1/2 cup of sunflower oil, 1 teaspoon of vanilla extract, and 1 cup of mango purée.

Combine Mixtures: Gradually pour the wet ingredients into the dry ingredients, folding gently with a spatula.

Bake Sponges: Preheat oven to 350°F (175°C). Divide batter into three 8-inch round cake pans and bake for 25-30 minutes.

Cool Layers: Allow cakes to cool in the pans for about 10 minutes before transferring to a wire rack to cool completely.

Prepare the Buttercream: Beat 1 cup of softened dairy-free butter until fluffy, gradually add 2 cups of powdered sugar and 1/4 cup of mango purée.

Assemble the Cake: Place the first layer on a serving plate, spread buttercream, drizzle with mango purée, repeat for second layer, top with last sponge, cover with more buttercream.

Decorate: Coat the sides and top of the cake with desiccated coconut and garnish with fresh mango slices and rosemary.