Preheat oven to 180 degrees C. Grease and flour one 25cm ring tin.
In a large bowl cream the butter with 1 cup of the white sugar. Beat in the eggs then the vanilla and the yoghurt.
In another bowl stir together the flour, bicarbonate of soda, baking powder and salt. Beat the flour mixture into the creamed ingredients. Turn half of the batter into another bowl.
Add the orange rind to one bowl and the melted chocolate to the other.
Alternately spoon the orange and chocolate batters into a the prepared ring tin. Swirl through the two batters with a knife to create a marbled effect.
Bake at 180 degrees C for 50 minutes. As soon as the done cake is removed from the oven stir together the remaining 1/4 cup sugar, the orange juice and the orange liqueur then pour over the still hot cake.
Let the cake finish cooling on a rack before serving.