Preheat the oven to 175°C / 350°F.
Cream the butter and both sugars in a stand mixer with a beater until light and fluffy. The colour should look pale, if there is still a yellow tinge to it, beat it for longer.
Add the egg and vanilla, mix until incorporated; then add the sourdough starter. Once that's added the mix should look like a thick cake batter.
Sieve the flour, baking soda, and salt in a bowl and whisk. I like to whisk the baking soda with the flour for even leavening distribution.*
Once they are combined, fold in the chocolate chips. The end mixture will look very thick and wet which is fine. Cover the dough with cling wrap and refrigerate for 20 minutes.*
With an ice cream scoop, (I used a No.16 / 81ml scoop) scoop into a baking tray lined with baking paper. Make sure to leave room as they tend to spread out. Bake for 15 minutes.*