Prep the pan and oven. Heat the oven to 325°F (165°C). Generously grease and flour a 10-inch bundt pan or two 8x4-inch loaf pans, tapping out excess flour. A well-prepped pan prevents sticking and ensures clean release.

Cream the butter and sugar. In a stand mixer with the paddle attachment (or with a hand mixer), beat the softened butter and sugar on medium speed for 4–5 minutes until pale and fluffy. Proper creaming builds structure and traps air for a fine crumb.

Add the eggs one at a time. Beat in the eggs, one by one, scraping the bowl after each addition. Mix just until incorporated. Overmixing here can toughen the crumb.

Stir in flavorings. Mix in the vanilla, lemon zest, and lemon juice (if using). These brighten the buttery richness without overpowering it.

Combine dry ingredients. In a separate bowl, whisk together the flour, baking powder, and salt. Keep it airy and evenly blended.

Add the sourdough and buttermilk. Stir the sourdough starter to loosen it. With the mixer on low, add the dry ingredients in three additions, alternating with the buttermilk (start and end with dry). Mix until just combined; a few small streaks are fine.

Final fold. Use a spatula to gently fold the batter, scraping the bottom and sides to catch any pockets of flour or butter. The batter should be thick, smooth, and silky.

Fill the pan. Spoon the batter into the prepared pan, smoothing the top. Tap the pan on the counter once or twice to settle the batter and release bubbles.

Bake. Bake for 60–75 minutes for a bundt (50–60 minutes for loaves), until a skewer comes out with a few moist crumbs and the top springs back lightly. If browning too quickly, tent with foil for the last 15 minutes.

Cool and release. Cool in the pan on a rack for 15 minutes. Turn out onto the rack and cool completely before slicing. Pound cake improves in texture as it rests.

Optional glaze. Whisk powdered sugar with lemon juice or milk to a pourable consistency. Drizzle over the cooled cake and let set before serving.