Hi Hilary! Thanks for the question. The answer here comes down to fat content, along with a few other things. Sour cream would add a higher percentage of fat to a recipe, creating a richer cake that would have a finer crumb. The buttermilk would create a lighter texture, perhaps not as fine of a crumb as something made with sour cream. Plus, the flavor can be a bit tangier with buttermilk, which can help cut the overall sweetness of a cake.

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