Orange Velvet Cake

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.