Step 1

Preheat oven to 180°C/350°F. Grease a deep 26.5cm x 33cm (10½-inch x 13¼-inch), 3.5-litre (14-cup) baking dish; line base with baking paper.

Step 2

Stir sugar, butter, sifted cocoa and soda, and the water in a medium saucepan over high heat, without boiling, until sugar dissolves. Bring to the boil. Reduce heat to low; simmer, uncovered, 5 minutes. Transfer mixture to a large bowl; cool to room temperature.

Step 3

Add flour and eggs to chocolate mixture; beat with electric mixer until mixture is smooth and paler in colour. Pour mixture into pan.

Step 4

Bake cake for 50 minutes. Stand cake in pan 10 minutes before turning, top-side up, onto wire rack to cool.

Step 5

Meanwhile, make fudge frosting. Stir butter, the water and caster sugar in a small saucepan over low heat, without boiling, until sugar dissolves. Sift icing sugar and cocoa into a small bowl, then gradually stir in hot butter mixture. Cover; refrigerate 20 minutes or until frosting thickens. Beat with a wooden spoon until spreadable.

Step 6

Spread cold cake with frosting.