Hi Samantha, if you'd like to add 100g of sourdough starter to this recipe, I think that would work fine. Reduce the milk by 50g and the flour by 50g, to account for the sourdough starter you're adding, and you might also consider reducing the baking powder to 1 teaspoon and increasing the baking soda to 3/4 teaspoon to adjust for the added acidity of the sourdough starter. If you'd prefer to leave the leavening as written in the recipe, it should still work. 

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