Preheat the oven to 350F with a rack in the center. Line a cookie sheet with parchment. Whisk eggs, sugar and vanilla in a small bowl. Set the bowl in a larger bowl of hot tap water until needed. In another bowl, whisk the flour and baking powder together. Stirring frequently, melt butter with 1 cup of the chocolate in the top of a double boiler over barely simmering water. Off heat, whisk in the egg mixture. Stir in the flour, nuts and remaining chocolate. Scoop slightly rounded tablespoons of batter 1 1/2 inches apart on baking sheet.
Bake until the surface of the cookie looks dry and faintly cracked, and the centers are still gooey, 10-12 minutes.
Set the pan on a rack to cool. Store cookies in a tightly sealed container.
PRODUCTS USED IN THIS RECIPE