Cream together the butter and sugars.
Add egg and mix through.
Mix in (by hand) the sourdough starter and the vanilla essence.
In a separate bowl, mix together the flour, baking powder, and baking soda.
Add the wet mixture to the dry ingredients.
Mix to incorporate all of the flour but try not to over mix.
Add in the chocolate chips and combine.
Rest the dough at room temperature for 15-30 minutes.
Preheat oven to 180℃ / 350℉.
Scoop small balls of dough (about 2 teaspoons per ball) onto a lined cookie sheet.
Flatten slightly with clean fingers or a fork.
Bake for 12-15 minutes until slightly brown at the edges.
Cool on a cooling rack.