In a medium mixing bowl whisk together the flour, salt and cocoa powder. Set aside
In a small measuring cup combine the vanilla and espresso powder. Mix until dissolved.
In a large mixing bowl beat the soft butter with an electric mixer for 1 minute or until smooth. Add the powdered sugar and brown sugar. Blend on medium until combined. Scrape down the sides and bottom of the bowl with a rubber spatula, and blend again.
Add the vanilla and mix just until incorporated.
Add the flour in three additions and mix on low speed until incorporated. The dough will seem a little dry, but keep mixing, it will come together.
Add the walnuts to the cookie dough and mix together using a wooden spoon, or mix by hand.
Place the dough on a large piece of wax or parchment paper. Form the dough into a log 12-inches long and about 1 1/2-inches thick. Wrap the dough tightly and refrigerate for at least 4 hours or overnight.