8 tablespoons (1 stick) unsalted butter cut into 1/2 inch cubes
9 ounces semisweet or bittersweet chocolate (no more than 62 percent cacao), finely chopped
3 ounces unsweetened chocolate, finely chopped
1/2 cup unbleached all-purpose flour
1/4 teaspoon fine sea salt
3 large eggs, at room temperature
1 1/4 cups superfine sugar
2 teaspoons pure vanilla extract
2 cups (12 ounces) semisweet chocolate chips
1 1/2 cups (5 1/2 ounces) coarsely chopped pecans
1 1/4 cups (4 1/2 ounces) coarsely chopped walnuts