Making the sponge
Preheat the oven to 175ºC.
Grease a traybake tin (33 cm x 23 cm) and line with parchment paper overlapping the edges.
Sift flour, bicarbonate of soda, cocoa powder and salt into a medium bowl and set aside.

Fit the Whisk Tool.
Add egg whites into the Appliance bowl.
Mix for 3 minutes on speed Max.
Transfer the whisked eggs into a large bowl.
Remove the Whisk Tool.

Fit the K-Beater.
Add butter and sugar into the Appliance bowl.
Tip: No need to clean the bowl.
Mix for 1 minute on speed 3.
Tip: Start the speed on Min and gradually increase speed.
Add oil, egg yolks, vanilla extract and vinegar.
Mix for 2 minutes on speed 3.
Add 120 ml buttermilk and half of the flour mixture.
Mix for 1 minute on speed 2.
Add the rest of the buttermilk and the flour mixture.
Mix for 1 minute on speed 2.
Add the red food colouring.
Mix for 1 minute on speed 4.
Using a spatula, gently fold in the egg whites.
Transfer the mixture into the tin.
Bake for 40-45 minutes at 175ºC, until a skewer inserted comes out clean.
Remove from the oven and let cool completely in the tin.
Tip: For decoration, trim off a small piece from the top of the cake after baking, crumble it and set the crumbs aside.

Making the cream cheese frosting
Clean the Appliance bowl and fit the K-Beater.
Add butter and icing sugar into the Appliance bowl and fit the Splashguard.
Mix for 2 minutes on speed 3, and then scrape down the sides of the bowl.
Tip: Start the speed on Min and gradually increase speed
Add cream cheese.
Mix for 2 minutes on speed 3.
Spread the frosting evenly over the top of the cake.
Sprinkle the reserved cake crumbs on top.

Serve.