In a bowl, add flour, cocoa powder, baking soda and salt. Whisk to combine.
In a separate bowl, beat softened butter and sweetener until light and fluffy. Add egg and vanilla, continuing to beat. Slowly beat in dry ingredients. Mixture will crumble and come apart – this is normal! Continue beating until it comes back together again.
Pour in dark chocolate chips and walnuts, and beat quickly just to distribute them throughout the dough.
Scoop cookie dough into balls on the baking sheet. Press down to form the cookie into a disk (cookies don’t spread much). Bake for 7 minutes. Remove from the oven before they look done, so they stay soft. They will stiffen up slightly as they cool completely. Enjoy!