Mix sourdough starter, flour, butter, and salt in a bowl to form a sticky dough. Split dough in half and wrap each half loosely in plastic wrap. Form into a flat rectangle-ish shape, transfer to the fridge and let chill for at least 30 min.
Preheat oven to 350F. While the oven is preheating, transfer one half of the dough to a lightly floured piece of parchment paper. Flour the dough lightly and roll out until thin. Transfer the parchment and dough to a baking sheet. Brush with some oil and sprinkle with flaky salt or any spice you want—paprika, za’atar, etc.
Prick the dough all over with a fork. Transfer to the oven and bake for approx. 25 min., turning the baking sheet halfway through. Repeat steps 2 and 3 with remaining dough. When crackers are cool, break into pieces and serve with your favorite dip (be careful though, these crackers aren’t super strong so dipping may cause breakage!) or meats and cheeses. Store airtight at room temperature for up to one week.