You'll want to work the vinegar into this buttery dough with a gentle touch, then sandwich it between sheets of parchment to roll it out. That trick keeps you from adding extra flour to the mix, which helps each shortbread bake through evenly.
Finish by showering the tops with sesame and nigella seeds, then bake until everything turns deeply golden and crisp. Be sure to let them rest on the baking sheet before moving them; they firm up nicely as they cool down. Those seeds give the rich, crumbly base a wonderfully nutty, savory punch.

