Preheat your oven to 350 ° (180 °C) and line your baking tray with parchment paper.
Beat the sugar and butter until it becomes light and fluffy.
Add the egg, sourdough starter, and vanilla extract and mix until completely combined.
When you have a smooth mixture, add the flour and salt and keep mixing until you have a soft, smooth dough.
Wrap the dough in plastic wrap and let it rest in the fridge for at least 30 minutes.
Roll out the dough on a well-floured surface and use your favorite cookie cutters to cut out your sourdough sugar cookies.
Place your cookies on a lined baking tray and bake them in the center of the oven for 10-13 minutes, or until golden. Because every oven is different, I recommend checking on your cookies every few minutes.
Let the cookies cool completely on a cooling rack before decorating.