• In the bowl of a stand mixer fitted with the paddle attachment, cream butter, sugar, and lemon zest. (You can also use a large bowl and hand mixer if you don't have a stand mixer).
• Add the sourdough starter, salt and vanilla and continue mix until fully combined.
• Slowly add the flour until just fully combined.
• Transfer the dough to clean work surface (no flour is necessary). Roll the dough into an approximate 14 inch cylinder.
• Pour the remaining 2 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.
• At this point, you can either proceed to bake the dough or chill it.

  To Chill: Cut the dough in half and wrap the pieces in plastic wrap. Chill in the refrigerator for up to 48 hours for a long fermentation.

When Ready to Bake

• Preheat oven to 375℉.
• Use a sharp knife to cut the dough into approximate 1/3 inch slices.
• Place the lemon cookies on a baking sheet lined with parchment paper, spacing 1 inch apart.
• Bake the cookies in the preheated oven for 12-15 minutes. The edges should just start to turn golden brown when done.
• Cool cookies in pan for 5 minutes, then transfer to a wire cooling rack to cool completely.

To Make the Lemon Glaze

• Whisk together the butter, powdered sugar, lemon juice and lemon zest in a small bowl.
• Place a sheet of parchment paper under your wire rack to catch any excess glaze that may drip off of the cookies.
• Dip half of each cookie in the glaze, allowing the excess to drip off. Return to wire rack. Sprinkle with extra lemon zest if desired.
• Allow the glaze to fully set before serving or storing.