Preheat oven to 350 degrees.
First, in a large mixing bowl, or in a bowl of a stand mixer with the whisk attachment, add the dry ingredients and whisk to combine.
Next, in a separate medium mixing bowl add the buttermilk, oil, eggs, pure vanilla, and sourdough starter discard.
Make sure to mix this well. You want the sourdough starter to be well combined with the other wet ingredients.
Add the wet ingredients to the dry ingredients and mix until well combined and no dry spots are left, scraping the sides of the bowl as you go. Make sure to not overmix as this will leave you with tough cake.
Grease, with cooking spray, a half baking sheet (18”x13”) well, making sure to get the sides of the pan. Then pour the cake batter into the pan.
Place in preheated oven and bake for 20-23 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool on a cooling rack and then frost or ice cake with Brown Sugar Frosting or frosting of choice.