Butter for greasing the pan • all-purpose flour • baking powder • fine sea salt • large eggs, separated and at room temperature • sugar • Finely grated zest of 2 lemons • neutral oil, like canola • Juice of 1 lemon (2 to 3 tablespoons) • pure vanilla extract • pure lemon extract or oil (optional) • About 1 1/2 cups (about 250 grams) blueberries, raspberries and/or blackberries (optional)