1/4 lb. (1 stick) unsalted butter, cut into 1/2-oz. pieces
1/2 cup all-purpose flour
1/4 cup unsweetened cocoa powder
1/4 tsp. baking powder
1/4 tsp. salt
4 large eggs
1 cup granulated sugar
2 Tbs. instant espresso powder
1/2 cup semisweet chocolate chips
1/4 cup pecans, toasted and coarsely chopped
1/4 cup walnuts, toasted and coarsely chopped
1 Tbs. pure vanilla extract
Preheat oven to 325F.
Melt chocolates and butter in top of double boiler or in medium-sized glass bowl in microwave oven, and stir until smooth. Set aside until needed.
In sifter, combine flour, cocoa powder, baking powder and salt. Sift onto large piece of parchment paper, and set aside until needed.
Place eggs, sugar and espresso powder in bowl of electric mixer fitted with paddle. Beat on medium for 4 minutes until mixture is thickened and slightly frothy. Add chocolate mixture, and mix on medium to combine, about 1 minute. Use rubber spatula to scrape down sides of bowl. Mix on low while gradually adding dry ingredients until incorporated, about 45 seconds. Scrape down sides of bowl.
Add chocolate chips, pecans, walnuts and vanilla extract. Mix on low to combine, about 10 seconds. Remove bowl from mixer, and use rubber spatula to finish mixing ingredients until thoroughly combined for batter-like consistency.
Using 3 heaping tablespoons of cookie batter per cookie, portion 6 cookies, evenly spaced, onto each of 3 nonstick baking sheets. Place 1 sheet on top rack and 1 sheet on center rack. Bake third sheet in separate oven, or hold at room temperature, then bake after first two sheets are removed.
Bake for 18 minutes total. After 9 minutes, rotate sheets 180 degrees and from top to center.
Remove cookies from oven, and cool to room temperature on sheets. Store cooled cookies in sealed container.