Step 1Position the oven racks in the center and upper third of the oven and heat the oven to 325 degrees. Line 4 large baking sheets with parchment paper.
Step 2In a large, heavy-bottomed saucepan, melt the semisweet chocolate and butter over medium-low heat until smooth, 5 to 7 minutes, stirring occasionally.
Step 3In a medium bowl, whisk together the flour, baking powder and salt.
Step 4In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, beat together the eggs, sugar, espresso powder and vanilla until blended and slightly foamy, 1 to 2 minutes. Beat in the melted chocolate mixture, then the flour mixture. Stir in the bittersweet chocolate and walnuts.
Step 5Using about 3 tablespoons of dough for each cookie, spoon the dough onto two of the prepared baking sheets, forming about 6 cookies per sheet. Bake until the cookies are cracked on top and moist in the center, about 14 minutes (do not overbake so as to keep the center of the cookies moist and fudgy). Transfer the baking sheets to a rack and cool the cookies completely before removing. Remove the cookies gently using a metal spatula.