Add flour, salt, espresso, and baking soda. Whisk, set aside.

Add sugars to standing mixer with paddle attachment and on low speed, combine for 1 minute. Add butter, 1 tbsp at a time to coat. Beat the butter and sugar until just creamy, scraping bowl as needed.

Keep mixer set to low, and add whisked egg. Add vanilla until blended. Add cider vinegar until blended. Stop mixing.

Add half the flour mix and blend on low until just combined, scraping as needed. Add the rest of the flour mixture, until the white just disappears (Overbeating leads to gluten formation.)

Add chocolate/toffee, mix on low for 5-10 seconds to distribute bits. Remove bowl from mixer, and gently fold to evenly distribute through dough. Put in an airtight container.

Refrigerate for 24 hours.

Remove from refrigerator, and bring dough to room temperature, about an hour.

Preheat oven to 365º and line a half-sheet pan with parchment.

Weigh 3 ounces of dough, 3 per sheet. Gently roll each into a ball, then tamp down until more like a disk. (This is necessary to get the crisp edge without charring.) Sprinkle a few grains of fine sea salt on each.

Bake for 15 minutes, rotating halfway through. Remove when the outside is darker brown, and the inside is lighter brown. Rest on sheet for 1 minute. Remove parchment sheet with cookies onto counter.. After another minute, move to cooling rack. (For ice cream sandwich cookies, double the vanilla and add 2 tbsp of water just before the cider vinegar.)