Line or grease and flour muffin cups, set aside.
In a large mixing bowl beat sugar and butter with electric mixer till well creamed. Beat in eggs and grated orange peel. Beat in sour cream and orange until combined.
In a smaller bowl mix together flour, baking powder, salt and soda. Mix chocolate into the flour mixture. Gently stir flour mixture (with chocolate) into the butter mixture until combined.
Spoon batter into the prepared muffin cups, filling each 3/4 full. Bake in a preheated 375 degree oven for about 20 minutes or until a wooden toothpick inserted comes out clean.