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Cream butter, cane sugar, and brown sugar in a mixing bowl until smooth.
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Add the egg and mix well.
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Mix in sourdough starter and vanilla extract until smooth.
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Sift in flour, baking soda, and salt. Mix until just combined.
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Fold in chocolate chunks.
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Scoop the dough onto a baking sheet (about 18 cookies total).
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Cover and refrigerate for at least 2 hours, preferably overnight.
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Preheat oven to 175°C (350°F). Bake cookies for 12–15 minutes.
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Let cool before serving. Store in an airtight container for up to 3 days or freeze unbaked dough for later.