Heat oven to 400. Spray a 12-cup muffin pan w/ cooking spray or use muffin liners.
In bowl, mix together flour, chocolate, baking powder, baking soda and salt. In another bowl, Cream together butter and sugar until smooth. Beat in eggs, one at a time.
Beat in orange zest, orange juice and buttermilk. Pour mix into flour mix. Mix until just moistened. Spoon batter into muffin cups, 3/4-full. Bake 15-20 min, or until a toothpick inserted in center comes out clean.