Melt the bittersweet chocolate chips and butter in a double boiler or in the microwave. Set aside to cool slightly.
Whisk together the flour and baking powder. Set aside.
In a large bowl, beat the eggs and sugar until thick. Stir in the chocolate mixture.
Stir in the flour mixture followed by the semisweet chocolate chips and nuts.
Use plastic wrap to help form the dough into two logs about 2 inches in diameter and 12 inches long. (The dough will be soft, so be sure to wrap well and tightly.)
Refrigerate the dough until firm (at least an hour up to overnight).
Preheat oven to 375°F (190°C). Line baking sheets with parchment paper or silicone liners.
Unwrap the chilled dough and cut into 3/4-inch slices with a sharp knife.
Place the cookies on the prepared pans, leaving about 2 inches between cookies.
Bake (one pan at a time) 12 to 14 minutes, or until a shiny crust forms on top but the interior is still soft. Keep the remaining dough refrigerated between batches.
Cool on the pans for 5-10 minutes. Then transfer the cookies to a wire rack to cool completely.
Notes
*At some point, Ghirardelli changed the size of their bittersweet chocolate chip packaging from 11.5 ounces to 10 ounces. Their updated recipe uses that smaller amount. I prefer them with the larger amount, but either should work. Other brands (like Guittard) still have the 11.5 ounce bags. Use another brand or roughly chop bar chocolate if you don't want to buy multiple bags.
While the slice-and-bake versions are pretty, I've also made these simply by dropping the dough with a scoop after the dough has chilled.