Step 1 Preheat oven to 180Â°C. Spray a 20cm-round cake tin with olive oil and line with baking paper. Place butter, sugar and vanilla extract into the small bowl of an electric mixer. Beat until light and creamy. Add eggs, one at a time, beating well after each addition.
Step 2 Transfer mixture to a large bowl. Sift flour with cocoa and bicarb soda. Add half the dry ingredients and half the milk to butter mixture. Gently fold together using a large metal spoon. Do not over-mix. Fold in remaining flour mixture and milk.
Step 3 Spoon mixture into cake tin and smooth top with the back of a wet spoon. Bake for 1 hour or until a skewer inserted into the centre of the cake comes out clean. Leave for 10 minutes then turn onto a wire rack to cool.
Step 4 To make frosting, combine marshmallows and milk in a microwave-safe bowl. Microwave on HIGH for 30 seconds, stirring every 10 seconds, or until smooth. Using an electric hand-beater, beat egg white and a pinch of salt until stiff peaks form. With beater motor running, slowly add cocoa to whites, beating continually until well combined. Fold marshmallow mixture into egg white. Stir until well combined and smooth. Refrigerate for 15-20 minutes or until firm enough to spread.