In a large bowl, whisk together the egg, sourdough discard, warm milk, vanilla, lemon zest, and sugar until well combined. It's okay if discard doesn't fully dissolve.
In another bowl add the flour, baking powder, baking soda, and salt. Whisk together dry ingredients, and add to your wet ingredients. Mix with a dough hook or spatula until a shaggy dough forms.
Add the softened butter in 1 Tablespoon pats all over the dough and knead with your hands for a few minutes until dough is smooth and butter is incorporated.
If desired, you can cover and refrigerate dough overnight for optimal fermentation. Otherwise, proceed with the recipe as listed below.