In double boiler over hot water, melt bittersweet chocolate chips and butter. In large bowl with electric mixer or whisk, beat eggs and sugar until thick; stir into chocolate mixture. In small bowl, stir together flour and baking powder; stir into chocolate mixture. Gently mix in semi-sweet chocolate chips and walnuts. Using a sheet of plastic wrap, form dough into two logs, each 2 inches in diameter and about 12 inches long. As dough will be quite soft, use plastic wrap to hold dough in log shape. Wrap tightly; refrigerate at least 1 hour or until firm. Heat oven to 375°F. Unwrap dough; with sharp knife, cut into 3/4-inch slices. Place slices 1 1/2 inches apart on greased or parchment-lined cookie sheet.