Stir together the egg, vanilla, melted butter, milk, and sourdough starter. Whisk until smooth and combined.
In a separate bowl, whisk together the flour, sugar, baking powder, baking soda and salt.
Add the dry ingredients to the wet ingredients, and whisk until just combined. Don't over mix. Batter will be thick, but if it's too thick add another tablespoon of milk. Set aside and let it rest for 20 minutes.
Meanwhile, place a skillet on the stovetop. When batter is nearly done resting, add a little bit of butter to the pan, and heat on medium. Ladle about 1/4 cup of the batter onto the skillet once hot, and cook on one side for about a minute or until set and bubbles form, then flip and cook on the other side for another 30 seconds to one minute.
Repeat with the rest of the batter. See recipe notes for tips on keeping the pancakes warm while you cook! Serve with pure maple syrup and any other toppings of your choice.