Position rack in center of oven and preheat to 350°F
Line rimmed baking sheet with parchment paper; butter parchment.
Whisk first 5 ingredients in large bowl to blend.
Add butter; rub in with fingertips until mixture resembles coarse meal.
Stir in chocolate and orange peel.
Whisk buttermilk and egg in medium bowl to blend; add to dry ingredients. Stir just until incorporated.
Turn dough out onto floured work surface and knead gently just until dough comes together, about 5 turns.
Form dough into 6 1/2-inch-diameter round, about 2 to 2 1/2 inches high. Transfer to prepared baking sheet. Using sharp knife, cut 1-inch-deep, 3-inch-long slits in top of bread, forming sunburst pattern.
Bake bread until well browned and very firm when pressed and tester inserted into center comes out clean, turning baking sheet halfway through baking, about 1 hour 10 minutes total.
Transfer bread to rack and cool completely, at least 3 hours. (Can be made 1 day ahead. Wrap in foil and store at room temperature.).