In a large mixing bowl, add the butter, sugar, vanilla extract and eggs and whisk together until well combined.
Now add the sourdough starter and whisk until the batter is light and marshmallowy.
Now, add all the dry ingredients to the bowl and stir together using a spatula until you have a thick, sticky cookie dough.
Cover the bowl with a plastic cover and place in the refrigerator for a minimum of two hours. It's fine to leave the dough up to 24 hours.
When you're ready to bake the sourdough carrot cake cookies, preheat your oven to 190C (375F) and line two large cookie sheets with parchment paper.
Use a cookie scoop to scoop out balls of dough and place them on the cookie sheets leaving 2" between each dough ball.
Take a flat bottomed glass or measuring cup and run it under water. Use the glass or cup to flatten each of the dough balls.
Bake the cookies for 20 minutes at 190C (375F) or until just golden. You want the edges to be crispy and the middles to be just set.
Allow the cookies to cool on the baking tray and then after 10 minutes transfer to a wire cooling rack to cool completely.