Cut butter up into chunks, place on a plate, and put in the freezer for 30 minutes to 1 hour.
Place butter and dry ingredients into a food processer and pulse until the butter is in small, pea-sized chunks.
Add in sourdough starter, vanilla, and lemon extract. If the dough is dry, add 1-2 tbs of cold milk or water. Start with 1 tbs and determine if it needs more.
Pulse until it starts to form a ball.
Place on parchment paper and shape into a square. Now you decide if you want to long ferment it or bake it right away.
If you are planning to bake it right away, you can add chocolate chips and nuts during the last pulsing phase. If you plan on using blueberries, fold into the dough when pressing it out.
To long ferment: Place in a covered dish and put into the fridge for 2-3 days. You can add chocolate chips or nuts before the long fermentation. I wouldn’t recommend adding blueberries before the fermentation process.
Preheat the oven to 425. Bake for 15-20 minutes.
Allow to cool. Add a glaze, if desired, or enjoy as is.