Melt the bittersweet chocolate chips and butter in the top of a double boiler or in a heatproof bowl over barely simmering water, stirring occasionally until smooth.
In a large bowl, beat the eggs and sugar with an electric mixer until thick; stir in the chocolate mixture.
In a small bowl, stir together the flour and baking powder; stir into the chocolate mixture. Gently mix in the chocolate chips and walnuts.
Using a sheet of plastic wrap, form the dough into two logs, each 2 inches in diameter and about 8 inches long. Because the dough will be quite soft, use the plastic wrap to hold the dough in the log shape. Wrap tightly; refrigerate for at least 1 hour or until firm.
Preheat the oven to 375 degrees F. Grease a cookie sheet or line with parchment paper.
Unwrap the dough. With a sharp knife, cut the dough into 3/4-inch slices.
Place the slices 1 1/2 inches apart on the prepared cookie sheet.
Bake for 12 to 14 minutes, until a shiny crust forms on top of the cookies but the interior is still soft.
Let cool on the cookie sheet. Then remove from the cookie sheet with a metal spatula.
Store in an airtight container at room temperature for up to one week.
