Combine the flour, sugar, baking powder, baking soda, salt, lemon zest and poppy seeds in the bowl of a stand mixer fitted with a flat beater (or in a large bowl if using a handheld mixer), and mix on low speed for 30 seconds.
Add unsalted butter and remaining sour cream, then resume mixing on low speed until just blended. Increase to medium speed and beat for another 1 1/2 minutes.
Storage: Keep airtight with plastic wrap at room temperature. Best if consumed within two days.
** Superfine sugar: If you don’t have superfine sugar, you can pulse regular white granulated sugar in a food processor.
This recipe was adapted from Rose’s Heavenly Cakes by Rose Levy Beranbaum.