Line an 8- or 9- inch (my reco) square pan with parchment paper.  I like to make a cross with two different pieces, making an easy sling to pull the baked shortbread out.

In a large mixing bowl (no stand mixer needed), sift the flour (regular and semolina), powdered sugar, and cocoa.

Add the salt, cinnamon, and brown sugar, then whisk or stir the mixture.

Add the avocado oil and the vanilla and switch to a spoon or rubber spatula. Mix until the dough is fully combined and there are no streaks or bits of flour, but then stop (overmixing will make it tough).  It may seem a bit oily but don't worry, it'll be fine.

Press the dough into the prepared pan, using the back of a measuring cup or the spatula to pack it in and smooth over the top.

Preheat the oven to 350 F.  Let the dough rest in the pan for 15 minutes while the oven heats up---trust me, this is important!  I skipped it accidentally once and my cookies were awful.  I believe that this is giving the flour and cocoa time to hydrate and the gluten time to rest and relax.

Once the oven is ready and the dough has rested 15 minutes, sprinkle the top with flaky sea salt and bake for 20 minutes.

Then remove from the oven.  Using a knife or scraper tool (my preference), cut the cookie in either half (for 16 cookies) or thirds (for 24 cookies), then turn the other way and cut it into 8 pieces (so in half, then in half, then half again if you prefer...).  Basically you want eight thin columns.  

Return to the oven for another 10 minutes, possibly up to 15.  Then remove from the oven and let cool.
Once the cookies are cool you can store them out in the air for a while (I like to dry them out a bit, even left them out on the counter overnight and they were fine) and then in an airtight container for up to a week.