Step 1

Preheat oven to 180°C. Brush a round 20cm (base measurement) cake pan with melted butter. Line base with non-stick baking paper. Stir the butter, caster sugar, orange rind and orange juice in a small saucepan over low heat until butter melts and sugar dissolves. Stir in the buttermilk. Set aside to cool.

Step 2

Sift flour into a bowl. Make a well in the centre. Stir in butter mixture and egg. Transfer 250ml (1 cup) of mixture to another bowl and sift over cocoa powder. Stir to combine. Pour the remaining mixture into prepared pan and spoon over chocolate mixture. Use a flat-bladed knife to create a swirled effect. Bake for 45 minutes or until a skewer inserted into the centre comes out clean. Set aside for 5 minutes to cool slightly before transferring to a wire rack to cool completely.

Step 3

Place orange in a small saucepan. Add enough cold water to cover and bring to the boil. Drain. Repeat 3 times. Stir the water and extra caster sugar in a saucepan over medium-high heat until sugar dissolves. Add orange and reduce heat to low. Simmer for 45 minutes or until the orange is translucent and the syrup thickens. Set aside to cool slightly. Transfer the orange to a wire rack over a baking tray.

Step 4

Use an electric beater to beat the butter in a bowl until pale and creamy. Gradually beat in the chocolate and icing sugar until smooth.

Step 5

Spread the icing over the cake and decorate with glazed orange.