Preheat oven to 350 degrees F. Lightly coat four 4- to 6-ounce ramekins with nonstick cooking spray. In a small bowl, stir together the 1 tablespoon granulated sugar and the espresso powder; sprinkle on the side and bottom of each ramekin. Place ramekins in a shallow baking pan; set aside.
In a small saucepan, stir together the 1/4 cup granulated sugar, the cocoa powder, flour, and salt. Gradually stir in milk. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir for 1 minute more. Remove from heat. Slightly beat egg yolk. Slowly add chocolate mixture to egg yolk, stirring constantly.
In a medium bowl, combine egg whites, vanilla, and cream of tartar. Beat with an electric mixer on high speed until soft peaks form (tips curl). Gradually add the 2 tablespoons granulated sugar, beating on high speed until stiff peaks form (tips stand straight). Gently fold chocolate mixture into egg whites. Spoon into prepared ramekins.
Bake about 25 minutes or until a knife inserted near centers comes out clean. Meanwhile, divide fudge topping among four shallow dessert bowls. Run a thin knife around edges and unmold each souffle into one of the prepared shallow bowls. Serve immediately with powdered sugar and walnuts.