In double boiler over hot water, melt the bittersweet chocolate chips and butter. In large bowl with electric mixer, beat eggs and sugar until thick; stir in chocolate mixture. In small bowl, stir together flour and baking powder. Stir into chocolate mixture. Gently mix in semi-sweet chocolate chips and walnuts.
On a sheet of plastic wrap, form dough into two logs, each 2 inches in diameter and about 8 inches long. Wrap tightly; refrigerate at least 1 hour or until firm. (Dough can also be frozen; thaw in refrigerator before proceeding with recipe.).
Preheat oven to 375 degrees. Unwrap dough. With sharp knife, cut into 3/4 inch slices. Place slices 1 1/2 inches apart on greased or parchment-lined baking sheet. Bake 12-14 minutes or until shiny crust forms on top but interior is still soft.