These are best within a couple of days of baking, as the texture doesn't hold up well over time, so you'll just have to eat a lot of them. The secret, of course, is the cacao nibs: little pieces of broken cocoa bean. Totally unsweetened, they give a texture much like nuts and a rich chocolate taste. Scharffen Berger makes them, and I found them for sale at Bread & Circus.
2 cups white flour
1/2 cup (gently packed) unsweetened cocoa
1/2 tsp. salt
1 tsp. baking soda
1 cup (2 sticks) unsalted butter, softened
3/4 cup white sugar
3/4 cup (packed) light brown sugar
2 eggs
1 Tbsp. vanilla extract
12 oz. bittersweet chocolate, chopped into largish chunks
12 oz. milk chocolate, chopped into largish chunks
1 1/4 cups cacao nibs
Sift together the flour, cocoa, salt, and baking soda in a medium bowl and set aside.
In a large bowl, cream the butter and sugar together. Beat in the eggs, one at a time, and then the vanilla. Stir in the flour mixture in three or four additions. Work the dough until it just starts to stiffen. (This will help the cookies to be puffy and chewy instead of flat and crispy.) Stir in the chocolates and the cacao nibs and mix until thoroughly combined.
Position a rack in the upper third of the oven and preheat the oven to 375°F. Drop the dough by thoroughly heaping tablespoonfuls (each one should be about 2 Tbsp. of dough) onto ungreased cookie sheets, leaving at least two inches between dough balls. Bake at 375°F for about 5 minutes, then reduce the heat to 350°F. Bake for 10-12 minutes more, until the cookies just hold together when lifted from the sheet with a spatula. They should be very soft and seem in imminent danger of falling apart. This is okay; they'll firm up as they cool. Cool on wire racks and store in airtight containers. Makes about 3 dozen hefty cookies.