2/3 cup Dutch processed cocoa
3 ¼ cups self-raising flour
2 cups firmly packed brown sugar
250g unsalted butter, melted, cooled
1 1/3 cups buttermilk
1 tablespoon vanilla extract
150g unsalted butter, chopped, at room temperature
3 cups icing sugar mixture
1/3 cup Dutch processed cocoa
¼ cup milk
Grease a 23cm x 33cm x 6cm deep roasting pan. Line base and long sides with baking paper, extending paper 3cm above pan edges.
Combine cocoa with 2/3 cup boiling water in a heatproof jug. Whisk until smooth. Cool.
Place flour, sugar, butter, eggs, buttermilk, vanilla and cocoa mixture in a large bowl of an electric mixer. Beat on low speed for about 1 minute, or until ingredients are just combined.
Increase speed to medium to high. Beat for a further 2 to 3 minutes, or until thick and smooth. Spoon into prepared pan. Smooth over top.
Cook in a moderately slow oven (160C) for 50 minutes to 1 hour, or until cake is cooked in the centre when tested with a skewer. Remove. Stand in pan for 20 minutes. Turn out onto a wire rack to cool completely.
To make frosting, beat butter in small bowl of an electric mixer until pale and creamy. Gradually beat in the combined sifted icing sugar and cocoa with milk. Continue beating until fluffy. Spread over cake.