• Preheat the oven to 350 F.
• Grease a 9x13 cake pan.
• In a large sized mixing bowl, whisk together the granulated sugar, milk, sourdough discard, eggs, and vanilla extract. Set aside.
• In a medium mixing bowl, whisk together the flour, baking powder, baking soda, and salt.
• To the flour mixture, add in the soft room temperature butter. Using your fingers, two forks, or a pastry blender, cut in the butter until it resembles coarse sand, with some pea sized butter remaining.
• Add the flour/butter mixture to the large mixing bowl. Using a hand held electric mixer, mix, mix for 3-5 minutes, until the batter is mostly lump free. Start on low speed and gradually work your way up to medium speed.
• Pour into the prepared 9x13 pan and spread out so the batter is even in the pan.
• Bake for 30-35 minutes, or until the cake is golden brown on top and a toothpick inserted into the center of the cake comes out clean.
• Allow to cool in the pan completely.

Frosting the cake

• In a medium mixing bowl, mix the room temperature butter with a handheld electric mixer until the butter is soft and fluffy.
• Add in the powdered sugar and mix to combine. I recommend starting with just ½ cup and mixing that, and then adding 1 cup, mixing, and then adding the final cup and mixing. That way the butter can slowly absorb the sugar and the sugar won’t fly all over your kitchen.
• Add in the vanilla extract, pinch of salt, and ¼ cup of milk. Mix until smooth.
• Taste and adjust for your preferences. If you want your frosting to be thicker, add a little more powdered sugar. If you want the frosting to be thinner and softer, add a splash more of milk. 
• Frost and decorate the cake as desired.