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.

Bake 12 to 14 minutes or until shiny crust forms on top but interior is still soft. Cool on baking sheet; store in airtight tin up to 1 week (if all of these delicious double chocolate cookies aren't eaten before then!)