Preheat oven to 180 degrees C (160 degrees C fan-forced). Grease a 3cm deep, 17.5cm x 27cm (base) slice tin. Line base and sides with baking paper, allowing 3cm overhang at long ends.
Combine sugar, butter, bicarbonate of soda, cocoa and 1 cup cold water in a saucepan over low heat. Cook, stirring, for 5 minutes until sugar dissolves. Bring to the boil. Reduce heat to medium. Simmer for 2 minutes or until mixture doubles. Transfer to a large bowl. Cool for 15 minutes.
Add flour and eggs. Using an electric mixer, beat mixture until pale and smooth. Pour mixture into prepared slice tin.
Cook in the preheated oven until a skewer into the centre comes out clean, about 45 minutes.
Frosting: Place butter, caster sugar and 2 tablespoons cold water in a saucepan over medium-high heat. Bring to the boil. Remove from heat. Sift icing sugar and cocoa into a bowl.
Gradually add butter mixture to the icing sugar mixture. Stir to combine. Cover. Refrigerate for 30 minutes or until frosting thickens. Beat with a wooden spoon until spreadable. Spread cake with frosting. Serve.