Grease a deep baking dish approximately 12” x 8” (30cm x 22cm) 4 quart (3.8 L) and line with baking paper.
In a saucepan melt the butter, sugar, water, cocoa and vanilla until sugar dissolves. Once sugar is dissolved bring to a boil and then simmer for 5 minutes. Be careful to not let the liquid boil over. Remove from heat and set aside to cool.
Once cool, preheat oven to 320 degrees fan (160C). Pour into a large mixing bowl and add flour and eggs. Beat with electric mixer until smooth and well combined.
Pour into the baking dish and place in preheated oven and bake for 45 – 50 minutes. Note; as ovens vary it may take up to 1 hour. Insert a skewer and if it comes out clean (without cake batter) it is ready.
Remove from oven, let stand for 5 minutes and remove from baking tray to wire rack to cool. Once cake is cool it is ready to frost and decorate.
Combine butter, water, superfine sugar and vanilla in a small saucepan to melt and dissolve sugar. Do not boil.
In a separate bowl, sift powdered sugar and cocoa.
Pour hot butter mixture into sifted powdered sugar and cocoa and stir well until combined. Allow the frosting to cool and thicken before spreading onto cake. It may help to place in the refrigerator.