Preheat oven to 325° F.
In a double boiler or a metal bowl set over a pan of barely simmering water melt unsweetened chocolate, 9 ounces semisweet chocolate, and butter, stirring occasionally, and remove top of double boiler or bowl from heat.
In a bowl with an electric mixer beat eggs with sugar until light and fluffy. Add espresso powder, vanilla extract and chocolate mixture, beating until smooth.
In a small bowl whisk together flour, baking powder, and salt and add to chocolate mixture, beating until just combined. Stir in remaining semisweet chocolate and nuts until combined well.
Scoop out 1/2-cup measures of cookie dough, arranging them 3 inches apart on ungreased baking sheets. Bake cookies in batches in middle of oven 25 minutes, or until tops begin to crack (do not overbake). Cool cookies on a rack.