Set oven to 350 degrees.
In KitchenAide Mixer, add butter, sugar, and eggs. Mix well.
Use a Lemon Juicer to squeeze the juice out of the lemon. Once juiced, add the juice to the batter and mix in gently.
Add sourdough starter to the batter, gently mixing it in.
In a separate bowl, add flour, baking powder, salt and lemon zest. Scoop the flour mixture into the batter as the KitchenAide is mixing slowly.
Add the milk and mix well.
Grease glass bread pan pour the batter into it.
Bake in oven for 50-60 minutes, or until the middle is well done.