Preheat the oven to 350F and line a baking sheet with parchment paper. In a large food processor fitted with an "S" blade, briefly process the walnuts into a crumbly texture.
Add in the dates, salt, baking soda, vanilla, flax, water, and vinegar. Process again, until the batter looks relatively smooth and sticky.
Add in the chocolate chips, or any other mix-ins you like, and process briefly to mix them in. Use a tablespoon or 1-ounce cookie scoop to scoop the dough.
Drop the cookie dough onto the lined baking sheet, and use your fingers to flatten and shape the cookies. (They don't spread much on their own.) I usually get about 16 to 18 small cookies from this batch.
Bake at 350ºF for 12 minutes, or until the edges are lightly golden. While the cookies are still warm, you can add a few more chocolate chips on top (if there are any spots that look bare). Let them cool completely on the pan, as they will be fragile while they are still warm. Store the cookies in an airtight container in the fridge or freezer, and serve cold for the most firm texture.
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Notes
Nutrition information is for 1 of 16 cookies. This will vary if you get more or less cookies out of the batch, and is just an estimate, not a guarantee.
I love the buttery flavor of these cookies when made with walnuts, but I have a feeling that pecans or almonds would work well, too.
If you don't have access to dates, raisins can be substituted in most cases, but the flavor will be slightly different.
If you don't want to use a flax egg, one chicken egg can be used instead.
Optional add-ins that are free of added sugar include: cacao nibs, raisins, shredded coconut, chopped nuts