• Heat the oven to 180°C conventional or 160°C fan-forced. Grease and line a 23 × 33 cm cake tin, leaving enough baking paper extending over two sides to lift out the cake.
• Place the flour, cocoa, baking powder, bicarb soda, salt and both sugars in a large mixing bowl. Whisk until evenly combined, breaking up any lumps of brown sugar or cocoa.
• Make a well in the centre. Add the milk, sour cream, oil, melted butter, eggs and vanilla. Stir gently until mostly combined.
• Gradually add the hot water, stirring just until the batter is smooth and evenly mixed. The batter should be quite fluid.
• Pour the batter into the prepared tin and gently tap the tin against the bench once or twice to release any large air bubbles.
• Bake for 25-30 minutes, checking at 25 minutes. The cake is ready when the centre springs back gently and a skewer comes out with a few moist crumbs but no wet batter.
• Leave the cake to cool for 10 minutes in the tin before transferring to a rack. Cool completely before icing.

Make the icing

• Stir the melted butter and cocoa together until smooth.
• Add the icing sugar, vanilla, salt and about three-quarters of the cream. Stir until smooth.
• Add the remaining cream a little at a time until the icing is thick but easily spreadable. If it is already soft enough, you may not need every drop.
• Spread the icing over the completely cooled cake. Add chocolate chips or sprinkles, if using, before the icing sets.