Step 1

Preheat oven to 170C/150C fan-forced. Grease a 3.5cm-deep, 18cm x 28cm (base) slice pan.

Step 2

Using an electric mixer, beat egg yolks, vanilla and sugar together for 5 minutes or until thick and creamy. Gradually add melted butter, beating until well combined. Add flour and cocoa, in 2 batches, beating until combined. Gradually beat in milk (mixture will be quite runny).

Step 3

Using an electric mixer and a clean bowl, beat egg whites and cream of tartar until stiff peaks form. Fold 1/2 the egg white mixture into chocolate mixture. Fold in remaining egg white mixture.

Step 4

Gently pour mixture into prepared pan (mixture will sit level with the top of pan). Bake for 40 to 45 minutes or until edges are just firm, but mixture is slightly wobbly in centre. Cool for 30 minutes. Refrigerate for at least 2 hours or until cold. Serve cake with whipped cream.