Preheat oven to 350°F (175°C). Grease and line a loaf pan with parchment paper.
Cream the butter and sugar together in a large mixing bowl until light and fluffy, about 2–3 minutes.
Add the eggs one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed.
Mix in sour cream, vanilla, and sourdough discard until smooth and well combined.
In a separate bowl, whisk together the flour and salt.
Gradually add the dry ingredients to the wet mixture, mixing on low speed or by hand just until combined — don’t overmix.
Pour the batter into the prepared loaf pan, smoothing the top.
Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean. If the top is browning too quickly, tent with foil halfway through baking.
Cool in the pan for 10–15 minutes, then transfer to a wire rack to cool completely before slicing.