Heat oven to 350°F. Place jumbo paper baking cup in each of 12 jumbo muffin cups.
In small bowl, mix flour, baking soda and 1/2 teaspoon salt; set aside.
In small microwavable bowl, microwave baking chocolate uncovered on High 1 to 2 minutes, stirring once, until softened and chocolate can be stirred smooth.
In large bowl, beat 1 cup butter until creamy.
Gradually add brown sugar and granulated sugar, beating well.
Add eggs, one at a time, beating after each addition.
Add melted chocolate and 1 teaspoon vanilla, beating well.
Add flour mixture alternately with buttermilk, beating on low speed after each addition until blended.
Divide batter evenly among muffin cups, filling three-fourths full.
Bake 30 minutes or until toothpick inserted in center comes out clean.
Cool 5 minutes; remove from pans to cooling racks. Cool completely, about 45 minutes.
frosting
In small microwavable bowl, microwave chocolate chips uncovered on High 1 to 2 minutes, stirring once, until softened and chips can be stirred smooth.
In large bowl, beat 1/2 cup butter with electric mixer on medium speed until creamy.
Add powdered sugar alternately with melted chocolate and whipping cream, beating on low speed after each addition until blended.