Sift the flours, cocoa and baking soda into a medium sized bowl. Whisk in the salt. Set aside.

Beat the butter with a mixer until creamy. Add the sugar and beat on medium speed until light and fluffy, about 3 minutes. Scrape down the sides and bottom of the bowl as needed (for example, if you see clumps of sugar not getting mixed in at the top of the bowl).

Beat in the egg yolks one at a time on medium low speed. Beat in the vanilla. Scrape down the sides and bottom of the bowl.

Add in the flour mixture and mix on low speed until the dough is mostly incorporated. Add the chocolate chips and then mix on low speed until completely incorporated.

Divide the dough into 4 roughly equal-sized pieces (do not stress over this much). Shape each piece into a log on a piece of plastic wrap and then roll up in the plastic. If you are baking later that day or the next day, place in the fridge for at least 3 hours. If you are baking a few weeks later, wrap the log in a layer of foil as well, and then place in the freezer.

When you are ready to bake, if you are using frozen logs be sure to give them 3 hours in the fridge to thaw.

Preheat the oven to 350 F.

Line a baking sheet with silicone or parchment paper (if you are baking a lot at once, line 2 baking sheets).

Using a sharp knife, slice your log into 1/4-inch thin rounds and place on the cookie sheet, 12 to a sheet. Sprinkle some coarse grey sea salt on each cookie.

Bake for 12 minutes, until the cookies have spread a little and look dry.

Let cool at least 10 minutes on the baking sheets--they will crisp up while cooling.

Store in airtight tins.