Sift the flour and kosher salt together in a large bowl and set aside. In an electric mixer with the paddle attachment, cream butter, brown sugar, vanilla, and espresso powder on medium-high speed until uniform, usually takes one minute.
Then, lower the speed on the stand mixer and add the flour and salt mixture slowly. Continue mixing until the brown sugar shortbread cookie dough starts to come together.
Finally, stir in the chopped walnut pieces. Now, form the mixture into cookie dough logs. About a 2-inch thick log should do. Wrap the log in plastic wrap and refrigerate for at least one hour before baking.
To bake cookies, preheat oven to 325 degrees. Line cookie sheet with parchment paper. Cut the cookie dough log into 1/4 inch thick slices and arrange on prepared baking sheet. These cookies don't spread much, so you don't need a ton of room between them.
Bake at 12-14 minutes, or until cookies are golden brown on the bottom. Cool cookies on sheet until firm and then transfer to a wire rack to complete cooling process. Store cooled cookies in an airtight container for up to 5 days.