Rub the inside of your baking dish with butter or brush oil all over. Add about 1 TBSP of flour and dust the whole dish. Discard any excess flour.
Preheat your oven to 350˚ F.
In a medium mixing bowl, whip the room temperature butter with a handheld mixer until very fluffy.
Add the sugar and mix on high speed for 3-5 minutes.
Add one egg at a time, mixing the batter on high speed after each addition.
Add the juice of about 1 ½ lemons, about 1 teaspoon of lemon zest, a few drops of lemon essential oil (if using), flour, baking powder, and the sourdough starter. Gently mix on low until combined. Do not over-mix!
Transfer the dough to the glass baking dish and spread it evenly (with a rubber spatula).
Bake for 35-50 minutes or until an inserted toothpick comes out clean.
Use the same toothpick to poke a lot of holes in the cake.
For the glaze, mix the juice of the remaining 1 ½ lemons, powdered sugar to taste, and a few drops of lemon essential oil (if using). Spread the glaze evenly over the cake.