Prep your pan and oven. Heat the oven to 350°F (175°C). Grease and line an 8-inch round or 9x5-inch loaf pan with parchment, leaving overhang for easy removal.

Combine dry ingredients. In a bowl, whisk flour, baking powder, baking soda, and salt. Set aside.

Beat butter, oil, and sugar. In a large bowl, beat softened butter, oil, and sugar until pale and fluffy, about 2–3 minutes. This adds air and helps the cake rise.

Add eggs and flavor. Beat in eggs one at a time. Stir in vanilla and lemon zest. Mix just until smooth.

Mix the liquids. In a small bowl or measuring cup, stir lemon juice into buttermilk or yogurt. It may look slightly curdled—that’s normal and helps with tenderness.

Combine wet and dry. Add the dry ingredients to the butter mixture in two additions, alternating with the lemon-buttermilk mixture. Start and end with dry. Mix on low until just combined. Do not overmix.

Prepare the berries. Toss blueberries with 1 tablespoon flour. This helps suspend them in the batter.

Fold in gently. Gently fold berries into the batter with a spatula. Keep a light hand so they don’t burst.

Fill and level. Spoon batter into the prepared pan and smooth the top. If using a loaf pan, mound slightly in the center to prevent doming.

Bake. Round pan: 35–45 minutes

Loaf pan: 50–65 minutes

Cool. Let the cake cool in the pan for 10–15 minutes, then lift out to a rack. Cool fully before glazing.

Glaze (optional but recommended). Whisk powdered sugar with lemon juice to a thick, pourable glaze. Adjust with more sugar or juice as needed. Drizzle over the cooled cake. Sprinkle with extra zest if you like.