1 stick (4 oz.) unsalted butter
7 bars (3.5 oz. each) semisweet chocolate –
12 oz. chopped, 7.5 oz. cut into 1-inch
squares and the rest for garnish
1 ½ cups flour
¼ cup unsweetened cocoa powder
1 ½ teaspoons baking powder
¼ teaspoon salt
1 ¼ cups light brown sugar
3 eggs, at room temperature
1 ½ teaspoons pure vanilla extract
30 marshmallows