Preheat the oven to 350 degrees F. Line a 9 by 5 baking pan with a piece of parchment paper.
To a large bowl, add the melted butter, lemon zest, granulated sugar and eggs. Whisk together until thoroughly combined. Add the freshly squeezed lemon juice and sourdough discard. Whisk again until completely combined.
To a small bowl add the flour, baking powder and salt. Fluff together.
Add the flour mixture to the lemon mixture and stir together. Add the sour cream and stir until completely combined. Pour the mixture in the parchment-lined baking pan.
Bake the loaf at 350ºF for 15 minutes. Then reduce the temperature to 325ºF for about 50 minutes. Stick a toothpick in the lemon loaf and if it comes out clean, it is finished baking. Let cool for 10 minutes before removing from the pan and continuing to cool on a wire rack. Let cool before topping with lemon glaze.
Lemon Glaze: Whisk together fresh lemon juice, powdered sugar and vanilla extract. Add some lemon zest if desired. Drizzle on top of the mostly-cooled loaf and enjoy.
