• In the bowl of a stand mixer using the paddle attachment, cream butter and sugar on medium speed.
• Add the sourdough starter, salt and vanilla and continue mix until fully combined.
• Set the stand mixer to low speed and slowly add the flour. Mix until just fully combined.
• Transfer the dough to a work surface. Use your hands to shape and roll the dough into an approximate 14 inch log. Try to roll it into a smooth cylinder with few wrinkles or creases.
• Pour the remaining 3 tablespoons of granulated sugar on the work surface along the long side of the shortbread dough. Roll the dough to completely coat it in the sugar.
• Cut the dough in half and wrap the pieces in plastic wrap. Chill in the refrigerator for 1 hour or up to 48 hours for a long ferment.

When Ready to Bake

• Preheat oven to 375℉.
• Line a large cookie sheet with parchment paper. Set aside.
• Use a sharp kitchen knife to cut the dough into approximate 1/3 inch discs.
• Place the shortbread cookies on the prepared baking sheet, spacing about 1 inch apart.
• Bake in preheated oven for 12-15 minutes or until the edges are just starting to turn golden brown.
• Allow to cool in pan for 5 minutes, then transfer to a wire rack to cool completely.