This recipe is from the Tartine (Chronicle Books, 2006), by our friends Elisabeth Prueitt and Chad Robertson, owners of Tartine Bakery in San Francisco. Be sure to take the cookies from the oven while they are still very soft to the touch because they will firm up as the cool.
Preheat the oven to 350 degrees F. Line baking sheets with parchment paper.
Melt the chocolate in the top of a double boiler or in a stainless steel bowl set over simmering water. Be sure that the water does not touch the bottom of the bowl. When the chocolate is melted, remove from the heat.
Stir together the flour, cocoa powder, and baking powder. Set aside. Using a standard mixer with the paddle attachment, beat the butter on medium-high speed until creamy. Slowly add the sugar and mix until the mixture is completely smooth and soft. Add the eggs, one at a time, mixing well after each addition. Scrape the sides of the bowl as needed. Beat in the salt and the vanilla, and then add the melted chocolate and beat on low speed until incorporated. Add the milk and beat until combined. Finally, add the flour mixture and beat on low speed until incorporated.
Drop the dough by heaping tablespoonfuls onto the prepared baking sheets, spacing them about 1 inch apart. Bake the cookies until they are just barely firm on top when lightly touched but are still very soft underneath, about 7 minutes. They will get firmer as they cool. Transfer the cookies to a wire rack and let cool. They will keep in an airtight container at room temperature for several days.