175g unsalted butter, chopped, at room temperature
1 ½ cups caster sugar
2 teaspoons finely grated orange rind
1 teaspoon orange food colouring gel
2 cups plain flour
2 teaspoons premium Dutch cocoa powder
1 cup buttermilk
1 teaspoon bicarbonate of soda
1 ½ teaspoons white vinegar
1½ cups icing sugar mixture
1 teaspoon finely grated orange rind
1-2 tablespoons orange juice
Orange food colouring gel
Grease a 20cm round cake pan (7cm deep). Line base and side with baking paper, extending paper 3cm above pan edge.
Beat butter, sugar and rind in a large bowl of an electric mixer until light and fluffy. Beat in colouring. Beat in eggs, one at a time, until combined. Add combined sifted flour and cocoa with buttermilk, in two batches, beating on a low speed until combined.
Combine soda and vinegar in a small bowl. Add to mixer. Beat for a further 2 minutes until smooth and thick. Pour into prepared pan. Smooth over top.
Cook in a moderately slow oven (160C) for about 1 hour and 10 minutes, or until cooked in the centre when tested with a skewer. Remove. Stand in pan for 30 minutes. Turn out onto a wire rack to cool.
To make icing, sift sugar into a bowl. Add rind. Gradually stir in enough juice until a smooth and a spreadable consistency. Tint with colouring. Spread icing over cake.
TIP! Un-iced cake can be made up to five days ahead. Store in an airtight container. Orange food colouring gel is available from cake decorating stores or online.