1. Chop 5 ounces bittersweet chocolate and 4 ounces unsweetened chocolate and place in a metal bowl; place over a small pot of simmering water and heat until melted, stirring occasionally. Chop remaining 4 ounces bittersweet chocolate into 1/2" pieces and set aside. Finely shave remaining 2 ounces unsweetened chocolate and set aside.
2. Add butter to metal bowl with melted chocolate and stir frequently until butter is melted; remove from heat and whisk in vanilla.
3. Combine sugar and eggs in a large bowl and beat with an electric mixer on medium until thickened and pale yellow, about 5 minutes. With mixer on low, slowly add melted chocolate and butter mixture and beat 15 more seconds.
4. In a medium bowl, combine flour, baking soda, cream of tartar, salt, espresso powder, reserved chopped bittersweet chocolate, reserved shaved unsweetened chocolate, and toasted walnuts. Fold dry ingredients into butter mixture with a rubber spatula until dough is well combined. Transfer dough to an airtight container and refrigerate for at least 3 hours and up to one day.
5. Place a rack in center of oven and heat to 350 degrees. Drop 1/4-cup portions of dough onto a baking sheet, set about 2" apart. Bake until cookies are cracked on top and centers are soft but not liquid-y when gently pressed, 15 to 20 minutes; let cool on baking sheet 10 to 15 minutes, then transfer to a wire rack to cool completely. Repeat with remaining cookie dough. Transfer cookies to an airtight container and store for up to three days.
Source: Hannaford fresh Magazine, December 2018