Recipe from Martha Stewart.
Preheat oven to 350°.
Heat chopped chocolate and butter in a microwave-safe bowl. Stir every 20 seconds until almost melted. Set aside.
Whisk together flour, baking powder, and salt. In a separate bowl, beat eggs, brown sugar, and vanilla at high speed with an electric mixer. Mix until light and fluffy. Beat in melted chocolate. Mix in dry ingredients until just combined. Stir in chocolate chunks.
Drop dough by heaping tablespoons onto baking sheets. Place 2-3 inches apart. Bake 12 to 15 minutes, or until cookies are shiny and crackly but soft in the center. Cool on baking sheets 10 minutes. Then, transfer to racks to cool completely.
Makes about 2 dozen cookies.