How to Make It
Prepare the Muffins: Preheat oven to 350°F. Place baking cups in 1 (12-cup) muffin pan.
Finely chop 2 ounces of the bittersweet chocolate; set aside remaining 6 ounces bittersweet chocolate. Place chopped chocolate in a small microwaveable bowl; microwave on HIGH until chocolate melts, about 1 minute, stirring every 20 seconds. Cool to room temperature, about 10 minutes.
Whisk together flour, cocoa, baking soda, and salt in a medium bowl.
Place butter in a large bowl; beat with an electric mixer on medium speed until creamy, about 1 minute. Add brown sugar; beat until fluffy, 3 to 4 minutes. Add egg; beat until incorporated. Add cooled melted chocolate; beat until incorporated.
Combine buttermilk and vanilla in a large bowl. Add flour mixture and buttermilk mixture to butter mixture in 5 additions, beginning and ending with flour mixture. Beat just until incorporated after each addition.
Coarsely chop remaining 6 ounces bittersweet chocolate; stir into batter. Divide batter evenly among prepared muffin cups.
Prepare the Streusel: Combine all Streusel ingredients in a medium bowl. Using your hands, crumble ingredients until small crumbs form and a streusel texture is achieved. Sprinkle evenly on muffin batter.
Bake in preheated oven until a wooden pick inserted in center of muffin comes out with moist crumbs, 20 to 25 minutes. Cool in pan on a wire rack 5 minutes. Remove from pan; cool completely on wire rack, about 20 minutes.