In the bowl of a stand mixer, cream together the butter and sugars until light and fluffy, about 5 minutes.
Add the vanilla extract and vinegar and mix an additional 2 minutes.
Sift together the flour, baking soda, cornstarch, and salt. Fold into the wet ingredients.
Add the sourdough starter and let the mixer run for 3-5 minutes until the sourdough starter is fully incorporated. Make sure the starter is FULLY incorporated or else the blondies could end up doughy in some parts.
Fold in the chocolate chips.
Pour the blondie batter into a parchment-lined 8"x8" baking pan and spread the mixture evenly. Bake for 23-25 minutes until golden brown across the whole top.
These blondies tend to sink a tiny bit right in the middle of the pan. But if they sink too much, they're underbaked.