In a medium-sized bowl, whisk together flour, cocoa powder, baking soda, espresso powder and salt. In a large mixing bowl, beat butter and sugars on medium-high speed until light and fluffy. Add eggs and vanilla extract and beat on high speed until combined. Gradually add flour mixture to butter-egg mixture and beat on low speed to combine. Beat in the chocolate chips and walnuts. Tightly cover bowl and chill dough in the refrigerator for 1-2 hours (or in the freezer for 20-30 minutes).
Preheat oven to 350°F (175°C) and line baking trays with parchment paper. Scoop 1 ½ tablespoons of dough, roll into balls (slightly smaller than golf balls) and place on baking trays, leaving space in between for cookies to expand.
Bake cookies for 8 minutes. You will see they are done when cracks appear on the surface of the cookies.rom oven and allow them set on the baking tray for 5 minutes before transferring them to wire racks to cool completely.