Directions
Place the warm water and dark rye flour into a 16-ounce glass jar and, using a fork, mix well to combine until a little sticky dough ball has formed. Once combined, place a lid on top of the jar but do not tighten. The dough may look a bit small for the jar but, it’ll need some room to grow. Leave the jar in a cool, dark place with a fairly constant temperature for two days. After day one, the mixture will look fairly similar what it looked like on the previous day.
Add the butter and all of the “sourdough starter” mixture into a stand mixer fitted with the paddle attachment. On a low speed, mix the softened butter with the sourdough until combined. Next, add in both sugars and cream together. Add in the egg and mix for a minute more. The mixture should look a bit more loose compared to most cookie doughs, that’s fine. Add in the vinegar and then the dry mixture little at a time until fully combined.
Using a one-ounce sized scoop, scoop the cookie dough onto a parchment lined sheet tray, leaving a few inches between each cookie. Bake for 6 minutes, rotate the cookie sheet, and bake for 6 minutes more. Remove from oven and top with a pinch of flaky salt. Repeat with the rest of the cookie dough, or freeze the cookie dough balls to bake later.