Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 10 inch tube pan.
In a large bowl, cream the butter with 1 cup of the white sugar. Beat in the eggs, then the vanilla and the yogurt.
In another bowl, stir together the flour, baking 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 greased and floured tube pan. Swirl through the two batters with a knife to create a marbled effect.
Bake at 350 degrees F (175 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 and pour over the still hot cake. Let the cake finish cooling on a rack before serving. Makes about 10 to 12 servings.