In a dry pan on low heat, gently toast the walnuts until fragrant- about 2-3 minutes. Do not burn. Set aside to cool completely.
In the bowl of a stand mixer fitted with a paddle attachment, cream butter until fluffy and glossy- about 3 minutes. Add in sugars. Beat on medium speed for 5 minutes to fully incorporate the sugar.
Add in eggs and vanilla paste. Beat another 2 minutes. Stir in espresso. Beat one minute more.
Combine flour, salt and baking soda in a small bowl. With the mixer on low, slowly add the flour mixture until fully incorporated.
Fold in the chocolate chips and toasted walnuts.
Cover the bowl and chill the dough for at least 5 hours or preferably overnight.
Pre heat oven to 400 degrees. Line baking sheet with parchment paper.
Using a large ice cream scoop, scoop out large balls of dough and place 4 inches apart on the cookie sheet. I only placed 4-5 cookies per sheet pan to allow for air circulation, even baking and proper rise of the cookies. Sprinkle with fleur del sel salt just prior to baking.
Bake at 400 degrees for 8 minutes, then reduce heat to 350 degrees for another 6-8 minutes or until the center is shiny but the edges have just turned golden brown. Baking time depends on your oven temperature so adjust as needed. Cool on a wire rack for 15 minutes. Serve immediately or store in an airtight bag up to 5 days.