After you have fed your sourdough starter, save the discard in a separate bowl. You should have about a cup. If you have a little more or less, that's fine. You can make up the difference with buttermilk.
In a large bowl, cream the butter and the sugar together on medium-high speed until light and fluffy. About 2 minutes using a hand mixer, or longer if you're using a whisk.
While mixing on low, add in the eggs one at a time and mix on medium speed until combined. (Remember to make sure they are room temp or even a little warm or they won't mix in properly.)
While mixing on low, add in the sourdough discard, buttermilk, baking powder, baking soda, salt, vanilla, and nutmeg. Mix until just combined.
Sprinkle in the flour and combine on low until the dough comes together. It will be sticky.