Adapted from Sarabeth's Bakery: From My Hands to Yours
Makes 24 cookies
1/2 cup (1 stick) butter
Position racks in the center and top third of the oven and preheat to 350 F. Line two baking sheets with parchment paper and set aside.
In a microwave safe bowl, combine butter, semisweet and unsweetened chocolate. Microwave mixture in 30 second intervals until mixture is melted and smooth. Cool slightly, until mixture is warm but not hot, about 5 minutes.
Sift the flour, baking powder and salt together into a medium bowl. In the bowl of an electric mixture fitted with the whisk attachment, beat eggs on medium high speed for about 30 seconds. Increase mixture speed to high and add the sugar and vanilla. Beat mixture until very thick, about 3 minutes. Reduce speed to medium and beat in the cooled chocolate mixture until fully incorporated. At this point change to the paddle attachment. With the mixer on low slowly add the flour mixture. Once fully mixed, add the chocolate chips, pecans, and walnuts (you may need to use a mixing spoon to make sure the add ins are evenly distributed).
Scoop out 2 inch diameter balls and place on baking sheets, spacing about 1 1/2" apart. Bake immediately, rotating pans from top to bottom and front to back halfway through, until cookies are set around the edges, 17-20 minutes. Do not over bake, cookies might seem soft in the center. Cool cookies completely on pans. Store at room temperature for up to 3 days.