Preheat the oven to 180°C, 160°C fan or gas mark 4. Grease the sides and line the base of two 8-inch round baking tins with greaseproof paper.
In a large bowl, add the flour, sugar, cocoa powder, baking powder and espresso powder. Use a whisk or fork to stir the ingredients together until well combined.
Now add the oat milk, sunflower oil, Oggs aquafaba and vanilla extract into the bowl. Beat the ingredients together, using an electric mixer set on medium speed, until smooth and combined
Reduce the speed and slowly pour in the boiling water bit by bit. Once all the boiling water has been added, turn the speed up to high and beat for 1 minute to add air into the mixture (little bubbles should start to form on top). Don’t worry if the mixture looks thin and runny, it’s meant to be like this!
Divide the mixture evenly between the two baking tins. Bake for 30-35 minutes, until the cakes have risen and the top is no longer shiny. Test the cakes by inserting a toothpick or skewer into the middle, if it comes out clean with only a few crumbs they are ready. Leave in the baking tins to cool completely
Once the cakes are cool, make the chocolate buttercream. Use an electric mixer to beat the Sainsbury’s baking block until soft, pale and lump-free. Now add the sieved icing sugar, cocoa powder, vanilla extract and oat milk and beat again until smooth
Now it’s time to decorate! Place one of the cake layers on a cake stand or plate. Use a spatula or palette knife to spread half of the chocolate buttercream on top
Add the second cake layer and spread over the remaining chocolate buttercream. Use the tip of the palette knife to create a swirl pattern in the buttercream. To finish, garnish the cake with fresh seasonal berries
Slice the cake into 12 and serve immediately, or store in the fridge in an airtight container for up to 3 days